Federal prosecutors say 2 U.S. nationals and 5 Russians conspired to feed Moscow’s ‘war machine.’

WASHINGTON — The United States is poised to approve sending its most advanced ground-based air defense system to Ukraine, responding to the country’s urgent request to help defend against an onslaught of Russian missile and drone attacks, two U.S. officials said on Tuesday.

Defense Secretary Lloyd J. Austin III could approve a directive as early as this week to transfer Patriot battery already overseas to Ukraine, the officials said, speaking on the condition of anonymity to discuss internal deliberations. Final approval would then rest with President Biden.

White House, Pentagon and State Department officials declined to comment on details of the transfer of a Patriot battery, which, if approved, would amount to one of the most sophisticated weapons the U.S. has provided Ukraine.

Ned Price, a State Department spokesman, told reporters on Tuesday that the United States would continue to prioritize sending air defense systems to help “our Ukrainian partners defend themselves from the brutal Russian aggression that we’ve seen for the better part of a year now.”

Many questions remain about the potential transfer, which was reported earlier by CNN, including how long it would take to train Ukrainian soldiers on the system, presumably in Germany; and where the Patriots would be deployed inside Ukraine.

Ukrainian officials have intensified their pleas for air defenses from the United States and other Western allies as Russia has conducted relentless attacks on power plants, heating systems and other energy infrastructure. The attacks, using missiles and Iranian-made drones, have left Ukrainians vulnerable and in the dark just as the coldest time of the year is beginning.

Over the weekend, Russian drone strikes on the southern Ukrainian port city of Odesa plunged more than 1.5 million people in the region into darkness. President Volodymyr Zelensky of Ukraine said the strikes by Russia, part of a nationwide assault on Ukraine’s energy grid, had left the region in a “very difficult” situation, warning that it would take days, not hours, to restore power to civilians.

In a speech to the Group of 7 nations on Monday, Mr. Zelensky thanked the countries for their continued support but listed financing for weapons first among his requests.

“Unfortunately, Russia still has the advantage in artillery and missiles,” he said. He requested additional artillery, as well as modern tanks — equipment that Ukraine has repeatedly asked for, along with fighter jets and longer range missiles.

The decision to send the Patriot system would be a powerful sign of the United States’ deepening military commitment to Ukraine.

The Pentagon’s active-duty Patriot units frequently deploy for missions around the world, and experts say there are not deep stockpiles of Patriot missiles available for transfer to Ukraine in the same way that the U.S. provided a large quantity of artillery shells and rockets to Kyiv for use in combat.

Five Russian citizens and two U.S. nationals were charged by federal prosecutors on Tuesday with conspiring to illegally obtain and ship millions of dollars worth of American-made, military-grade technology to Russia before and after its invasion of Ukraine this year.

The case, described in an indictment unsealed in federal court in Brooklyn, accuses the seven defendants of acting on behalf of two Moscow companies, Serniya Engineering and Sertal LLC, that prosecutors say operate under the direction of Russia’s intelligence services.

Starting in at least 2017, the indictment says, the defendants used a global network of shell companies and bank accounts to acquire and ship advanced electronics and equipment to Russia that can be used for “quantum computing, hypersonic and nuclear weapons development and other military and space-based military applications.”

Among those items, according to the indictment, were a “chip set” of 45 advanced semiconductors; a $45,000 “low noise cesium frequency synthesizer”; high-quality spectrum analyzers used in electronic testing; and pricey oscilloscopes and signal generators. Many are so-called dual-use devices that can be employed for military or civilian purposes.

The illicit activities continued after the United States imposed sanctions against Serniya, Sertal and one of the defendants, Yevgeniy Grinin, following Russia’s invasion of Ukraine in February, according to the Justice Department.

When it announced the sanctions, the Treasury’s Office of Foreign Assets Control said those being targeted were “instrumental to the Russian Federation’s war machine.” Breon Peace, the U.S. attorney in Brooklyn, echoed that language in announcing the charges on Tuesday.

“Our office will not rest in its vigorous pursuit of persons who unlawfully procure U.S. technology to be used in furtherance of Russia’s brutal war on democracy,” Mr. Peace said.

In addition to Mr. Grinin, a 47-year-old Moscow resident, the Justice Department identified the defendants as Alexey Ippolitov, 57, of Moscow; Boris Livshits, 52, a former Brooklyn resident now living in St. Petersburg, Russia; Svetlana Skvortsova, 41, of Moscow; Vadim Konoshchenok, 48, of Tallin, Estonia; Alexey Brayman, 35, of Merrimack, N.H.; and Vadim Yermolenko, 41, of Upper Saddle River, N.J.

The defendants face various conspiracy, fraud and money laundering counts, and could be sentenced to up to 30 years in prison apiece if convicted of the most serious charges. Four of them remain at large.

Mr. Yermolenko, an American citizen, and Mr. Brayman, a permanent U.S. resident identified in court filings as an Israeli citizen, were arraigned on Tuesday.

Mr. Brayman was released on a $150,000 bond and subject to conditions that included turning in his passports and wearing an electronic monitoring device. “Like all defendants, Mr. Brayman is entitled to the presumption of innocence,” said his lawyer, David Lazarus.

Mr. Yermolenko was released after posting a $500,000 bond partially secured by his house. Nora Hirozawa, a lawyer for Mr. Yermolenko at his arraignment, declined to comment.

Mr. Konoschenok, who the U.S. authorities believe is an officer in Russia’s domestic intelligence agency, the Federal Security Service, was arrested in Estonia last week and is awaiting extradition proceedings, according to prosecutors. Estonian authorities found 375 pounds of U.S.-made ammunition in a search of a warehouse he has used, the Justice Department said.

Glenn Thrush contributed reporting.

PARIS — International leaders agreed to deliver about $1 billion in fresh financial aid for Ukraine on Tuesday to rapidly repair energy grids, water systems, roads and health centers decimated by relentless Russian strikes, the latest attempt to buoy Ukraine through what is already a brutal and dark winter.

The pledge of aid came at a one-day summit in Paris convened by President Emmanuel Macron that brought in countries outside of the usual list of Ukrainian allies. Leaders from about 50 countries attended, including Cambodia, Saudi Arabia, Bahrain, Turkey, Kuwait and Oman.

“It’s tangible proof Ukraine is not alone,” Mr. Macron said at the opening of the summit. He was flanked by Denys Shmyhal, Ukraine’s prime minister, and Olena Zelenska, Ukraine’s first lady. Leaders and diplomats from about 50 countries also attended.

“The fight you are waging is a fight for your freedom, your sovereignty,” Mr. Macron said. “But it is also a fight for the international order and for the stability of all of us.”

The Paris conference was the latest in a series of international meetings focused on the current and future reconstruction of Ukraine. On Monday, leaders from the Group of 7 wealthy democracies met virtually to agree a new system of funneling funds to Kyiv, and the European Union’s foreign ministers met in Brussels, pledging another €2 billion for military support. Weeks before, the United States pledged $53 million to rebuild Ukraine’s electricity grid at a NATO meeting in Bucharest.

What makes Tuesday’s conference and announcements different, organizers said, was the timeline. The money raised will be delivered between now and the end of March to meet Ukraine’s short-term needs. One criticism of previous aid pledges has been the length of time for delivery.

The French leader also announced the creation of the “Paris mechanism” — a platform designed to ensure that donors coordinate urgent deliveries and match them to Ukrainian needs. Hubs in Poland and several other countries will collect international donations, including generators and heat pumps, that can be swiftly shuttled into Ukraine.

President Volodymyr Zelensky, speaking from Ukraine at the conference’s opening via video feed, said that reversing the extensive damage to his country’s energy infrastructure would cost about €800 million ($840 million) and predicted that Russia was likely to “intensify its attacks” during the winter.

Ukraine’s government has estimated that it would cost $750 billion to rebuild the war-battered country, although the World Bank has put that estimate at closer to $349 billion.

Officials gathered at the Paris meeting pledged to support Ukraine for the long haul, and many reiterated the argument that Russia’s attacks on civilian infrastructure amounted to war crimes.

​​“I’m not the biggest at the table,” said Prime Minister Xavier Bettel of Luxembourg, committing €4 million in immediate aid. “But you can count on us. We are with you.”

The Swiss Parliament approved the immediate transfer of 100 million Swiss Francs ($107 million), said President Ignazio Cassis of Switzerland. Mr. Macron committed €76.5 million in immediate assistance from France, in addition to €48.5 million already pledged.

KYIV, Ukraine — Ukraine is stepping up efforts to isolate and degrade Russian forces in and around Melitopol, a city in the Zaporizhzhia region that would figure prominently in any Ukrainian offensive to drive Russian forces from southern Ukraine.

Kyiv has been using long-range precision missile strikes, sabotage missions and targeted assassinations to put pressure on the city, which is known as the “gateway to Crimea” because of its location at the crossroads of two major highways and a vital rail line. After one series of strikes over the weekend, Ukraine claimed to have destroyed an entire Russian Army command.

Officials in both countries have acknowledged the recent attacks on Russian command centers, ammunition depots and supply routes in Melitopol, whose prewar population was about 150,000. The aftermath of some of the attacks have been captured on video shared on social media by Russian soldiers.

It is not clear whether the strikes were the prelude to an offensive or a deceptive ploy as Ukrainian forces prepare to move on Russian forces elsewhere. But military analysts described them as significant and said they fit Ukraine’s pattern of using precision missiles to strike Russian logistical targets.

Melitopol is a key hub, and regaining control over it could help Ukraine’s forces take back Russian-held areas in the Zaporizhzhia region and Kherson regions. That could then potentially give them a path to drive Russian forces back to Crimea.

“All this hangs completely on Melitopol,” Oleksiy Arestovych, a top adviser to President Volodymyr Zelensky, said in his daily podcast. “If Melitopol falls, the entire Russian defense up to Kherson collapses, and the Ukrainian armed forces ‘jump’ right to the border with Crimea.”

On Monday night, a bridge in Melitopol leading over the Molochna River was sabotaged by Ukraine’s forces, Ukrainian and Russian officials said. Video showed two pillars underneath the span had been blown up, compromising a key Russian supply route to Melitopol.

A day earlier, the Melitopol Christian Church — which the city’s exiled mayor said was being used as a Russian base — had gone up in flames.

That came after residents reported at least 10 large explosions on Saturday night and Sunday morning, although it was not clear whether some of those were Russian air defenses at work.

On Saturday night, several blasts hit a hotel and restaurant complex on the outskirts of the city, according to Ukrainian officials and video of the aftermath. The mayor, Ivan Fedorov, said that the facility, known as the Hunter’s Halt, was being used by Russian intelligence and that dozens of Russian soldiers had been killed, with hundreds more wounded and evacuated to Crimea for medical care. The Ukrainian military high command said Tuesday night that the strike using precision HIMAR missiles to “destroy” the entire command of the 58th Russian Army, which was gathered at the facility. The claim could not be immediately verified.

Evgeny Balitsky, the Russian-appointed head of the part of the Zaporizhzhia region that Russia claimed to annex in September, said that the facility had been hit while “peaceful citizens” were eating on Saturday evening. He said that two people had been killed and 10 injured.

Ukraine has also targeted members of the local Russian occupation administration. Nikolai Volyk, the deputy head of the occupation government in Melitopol, survived an bomb attack outside his home last week, the Russian state-run news agency RIA Novosti reported.

On Tuesday afternoon, a loud explosion was reported in the center of the city, Mr. Federov said in a statement. The explosion was followed by sustained gunfire, he said. It was not immediately clear what might have been targeted.

The United Nations nuclear watchdog agency will maintain a continuous presence at all of Ukraine’s nuclear power stations, including the defunct Chernobyl plant, as Moscow takes aim at energy infrastructure across the country, the agency and the prime minister of Ukraine said on Tuesday.

The nuclear watchdog, the International Atomic Energy Agency, has for months had inspectors at the Russian-held Zaporizhzhia nuclear plant, where frontline fighting has heightened the risk of a nuclear accident.

Now the I.A.E.A. will install permanent teams of safety and security experts at four other plants at a time when Russian forces have repeatedly bombarded Ukraine’s energy grid, leaving millions without power as cold weather has begun to set in.

With the Zaporizhzhia nuclear plant — Europe’s largest — under Russian control, Ukraine is heavily reliant on three other plants — two in the western Ukrainian cities of Khmelnytskyi and Rivne, and one in the south near the city of Yuzhnoukrainsk.

Last month, Russian missiles knocked the Khmelnytskyi plant offline, and the Rivne plant had to reduce its output because the power lines that connect it to the national grid were damaged by strikes.

The Chernobyl plant, the site of the world’s worst nuclear accident in 1986, was shuttered after the meltdown but still requires electricity to power cooling equipment.

The Zaporizhzhia plant’s reactors have been shut down since September, but the plant remains the greatest concern among nuclear experts. Shelling has repeatedly disrupted its connection to outside power, forcing it to rely on diesel generators for cooling. Each country blames the other for the damage.

The director general of the I.A.E.A., Rafael Mariano Grossi, has spent months trying to secure a cease-fire around the Zaporizhzhia plant, where Ukrainian officials say Russian forces have stationed heavy weapons, a claim that Moscow denies.

Mr. Grossi said he was “increasingly optimistic” about the prospect of negotiating a demilitarized zone around the plant after meeting on Tuesday with Ukraine’s prime minister, Denys Shmyhal, and its energy minister, Herman Halushchenko, at a summit in Paris.

Ukrainian energy officials recently accused Russian forces of installing rocket launchers at the plant. On Tuesday, Dmitri S. Peskov, a Kremlin spokesman, denied that Russia has heavy weapons stationed at the plant. He said Moscow appreciates its level of interaction with the I.A.E.A. and remains in contact with the agency on issues concerning the Zaporizhzhia plant.

KYIV, Ukraine — The importance of Melitopol is no secret. It was one of the first cities that Russian forces seized early in the war, and it has been central to their strategy.

After Russian forces were routed in northeastern Ukraine and forced to retreat from the southern port city of Kherson, they tried to fortify positions on the stretch of land that runs to the Crimean Peninsula from Russia — the Kremlin’s coveted “land bridge.” They have paid special attention to Melitopol, which is at the crossroads of two major highways and a vital rail line.

Natalia, a retiree who lives in Melitopol, said she started to see a large influx of Russian soldiers in the southern city in late November. They took over schools, she said, and she saw Russian soldiers moving in weapons and heavy artillery to positions in and around the city.

“So many of them everywhere,” said Natalia, who asked that her family name be withheld out of concern for her safety.

Natalia also has a house outside the city and said she had watched as Russian planes and helicopters flew so low over the area that she could see “Z” painted on them, referring to the letter that has become a symbol of support for the invasion.

“I have a headache from this constant noise,” she said.

In early December, Russian forces nearly closed all access in and out of the city, said Ivan Fedorov, the city’s exiled mayor.

In a hunt for partisans — guerrilla fighters and others working for the Ukrainian war effort — Russian forces have set up several roadblocks and are searching cars and people, Mr. Fedorov said.

After each explosion, Mr. Federov said, Russian forces go house to house. If they found any Ukrainian symbols or weapons, even hunting ones, people were taken for questioning or held, he said.

Many prisoners, he said, are now being forced to help dig trenches around Melitopol.

Three Ukrainian ports on the Black Sea were back up and running again on Tuesday, three days after Russian strikes on the southern city of Odesa caused major blackouts across the region and forced the ports to shut down, officials said.

The Russian strikes, carried out by drones, pounded energy infrastructure in Odesa over the weekend, leaving 1.5 million people without power — a situation that President Volodymyr Zelenskywarned would take “days” to rectify.

Power to the three ports — the only Ukrainian ports on the Black Sea that are still in operation — was also disrupted, but operations returned to normal on Tuesday, according to the Ukrainian Sea Ports Authority.

“Port workers resumed loading ships with export agricultural products,” Olena Hiriayeva, a spokeswoman for the organization, told the Ukrinform news agency.

The three ports are all located in the Odesa region: in Odesa itself, in Chornomorsk to the west and the Pivdenniy port in Yuzhne to the east.

They are open under an agreement between Russia and Ukraine that allows for the shipment of Ukrainian grain through a humanitarian corridor to Turkey.

The deal, which was brokered by the United Nations and Turkey in July to end a monthslong Russian blockade, was intended to ease global food shortages and is seen as a rare example of diplomacy between the two countries.

Repair crews were able to restore some power to the ports in Chornomorsk and Yuzhne by Sunday, but it took longer to restore service to the port in Odesa, according to the port authority.

No vessels left the ports on Monday because of “unfavorable weather conditions and electricity shortages,” according to the U.N. agency that tracks the vessels leaving Ukraine. Seven vessels carrying agricultural products left the ports on Tuesday, carrying more than 180,000 tons of agricultural products, according to U.N. data.

BRUSSELS — Ambassadors from European Union countries have reached a complex deal to provide 18 billion euros ($19 billion) in loans to Ukraine, according to the Czech delegation running the negotiations, a significant lifeline that had been ensnared in a long-running fight between the full bloc and the Hungarian government over corruption and the rule of law.

To approve the funding, which required unanimous support from all E.U. countries, the ambassadors agreed to give Hungary access to some of its own European aid money in exchange for promised overhauls of its political and judicial systems. For months, over €13 billion of the bloc’s funding for Hungary has been practically frozen, and to get access to some of it, Hungary’s government has been wielding its veto over several E.U. decisions that require unanimous support.

Under the agreement, Hungary’s access to the bloc’s coronavirus funds will be disbursed if Budapest introduces overhauls such as establishing an independent anticorruption body and an integrity authority. Those funds would have been lost if the bloc had not approved Hungary’s spending plan for them by the end of this year.

The agreement would also lower — to €6.3 billion, rather than €7.5 billion — the amount of Hungary’s funds that the European Commission, the bloc’s executive arm, has proposed to freeze from the bloc’s regular budget because of Hungary’s rule-of-law backsliding in recent years. The remainder of that money can be unfrozen if the Commission and member nations deem that Budapest has made sufficient progress on the overhauls.

The bloc’s current budget, which runs from 2021 to 2027 and is separate from the coronavirus recovery fund, includes €34.5 billion for Hungary.

Hungary’s standoff with the European Union goes back years, as Prime Minister Viktor Orban, a Russia-friendly strongman, has increasingly reshaped his country’s institutions, and whittled away at democracy, to suit his will. But the conflict recently escalated when the European Commission recommended freezing a significant portion of the bloc’s regular budget for Hungary and blocked Hungary’s access to the E.U. coronavirus recovery fund amid the dispute.

To get past the stalemate, the ambassadors of E.U. countries agreed late Monday to approve Hungary’s spending plan for the coronavirus funds. In return, Hungary, which is being squeezed by a severe economic crisis, has agreed to drop its veto on the aid package for Ukraine.

The decision is expected to be made final by the bloc’s ministers in a written procedure on Wednesday.

The agreement has been called a “megadeal” by the Czech government, which currently holds the bloc’s presidency.

Belarus said on Tuesday that its army was assessing its combat readiness, a move that comes as the country has faced pressure to provide further support for Russia’s forces in Ukraine.

Military experts say it is highly unlikely that Belarus will send troops to Ukraine, not least because it would be deeply unpopular domestically, but they say that President Aleksandr G. Lukashenko of Belarus may be giving the impression of combat readiness in order to force Ukraine to divert troops from other fronts.

Mr. Lukashenko is a close ally of President Vladimir V. Putin of Russia, and Belarus relies on Moscow for finance, fuel and security assistance. Moscow used eastern Belarus as a staging ground when it launched its full-scale invasion of Ukraine in February. The United States and European governments have imposed sanctions on both countries.

A Belarusian Defense Ministry statement said that “a sudden check of combat readiness has begun” on the orders of Mr. Lukashenko.

As part of its preparations, Belarus’s troops will have to move to predetermined locations and test their equipment, organize security and defense and build bridges across the Neman and Berezina rivers, the Defense Ministry said on the Telegram messaging app.

The Neman river runs through Lithuania and eventually drains into the Baltic Sea, while the Berezina joins the Dnipro River around 40 miles north of the Ukrainian border. The exercises would not directly approach the frontier between Belarus and Ukraine, which is around 670 miles long.

In October, Mr. Lukashenko said that Russian troops would return to his country in large numbers, and in recent weeks drones have been launched from Belarus against Ukraine’s energy infrastructure and other targets. In addition, supplies of military equipment have been ferried by Russian forces from Belarus to their troops in eastern Ukraine’s Donbas region.

Ukrainian forces are already fighting in the south and east. In light of the potential threat from the north, Andriy Demchenko, a spokesman of Ukraine’s State Border Guard Service, said on Tuesday that the country was strengthening its defenses along the length of its border with Belarus. He said the situation is under control.

“We can clearly see how Russia is putting pressure on Belarus to join the full-scale war,” he said on a Ukrainian telethon.

Ukraine in recent weeks began to build a wall and trench system along the frontier in the northwestern province of Volyn, though analysts say that it is more likely to stop migrants crossing rather than for a military purpose.

The Institute for the Study of War, a research group based in Washington, said in a report this week that Belarus is “extraordinarily unlikely to invade Ukraine in the foreseeable future.”

Instead, it said that Mr. Lukashenko and other officials in Belarus have assisted with an effort by Moscow to suggest that Belarus will join the war directly in an effort to pin down Ukrainian troops on their northern border.

At the same time, the Institute said that Moscow aimed to bind Belarus further into its campaign in Ukraine “as part of a long-term effort to cement further control over Belarus.”

KYIV, Ukraine — Many Ukrainians saw messages of defiance and resilience in the seven artworks painted last month by Banksy, the British street artist, on war-ravaged buildings in and around Kyiv.

At least one activist saw another kind of benefit: He removed one of the works, saying he intended to auction it and donate the proceeds to the Ukrainian Army.

The activist, Serhiy Dovhyi, says he is now under criminal investigation for removing the work from a wall in the Kyiv suburb of Hostomel. The depiction, of a woman in a bathrobe wearing a gas mask and holding a fire extinguisher, suggests the war’s intrusion on home life. A Ukrainian art dealer has estimated that the work is worth up to $1 million.

Under wartime authorities, the Ukrainian military appoints local leaders. The military-appointed head of Hostomel told local media that the art should go into a future memorial to war or remain on the site, to become part of a future building.

It isn’t the first time ownership of one of Banksy’s works has been in dispute. In 2014, a Banksy painting appeared on a piece of plywood secured to the Broad Plain Boys Club in Bristol, England. The club’s owner, Dennis Stinchcombe, planned to auction the painting to raise money for the club, but the city stepped in and claimed it owned the depiction of a couple embracing and staring at their cellphones. In a rare public move, Banksy wrote a letter saying the art should be used to help the club.

In the Ukraine dispute, Mr. Dovhyi said in an interview that the artwork had to be saved because the wall on which it was painted was scheduled to be demolished soon. He described the act of removing the graffiti, which he documented in videos, as an additional act of performance art that might add to its value.

Mr. Dovhyi cut the graffiti from the wall in Hostomel on Dec. 2 by removing a layer of insulation on the ruined building’s exterior. .

“Street art, in contrast to a piece of art in the Louvre, doesn’t belong to anyone,” he said.

The action did, in any case, succeed in stirring discussion in Ukraine about the future of Banksy’s local works. The art appeared in the suburban towns Hostomel, Irpin and Borodyanka, where hundreds of civilians died early in the war in air and artillery strikes and summary executions.

For nearly two decades, Banksy has maintained his anonymity, while creating art, often with social and political undertones, in New York City, London, the West Bank and many other places.

Banksy’s work has also included stunts. In 2018, his painting “Girl With Balloon” self-destructed in a remote-controlled shredder moments after Sotheby’s in London auctioned it for $1.4 million. The apparent commentary on the excesses of the art market only enhanced the work’s value: Retitled “Love Is in the Bin,” Sotheby’s resold it last year for $25.4 million.

But Banksy also goes to great lengths to regulate the resale of his graffiti and prevent fakes, working with a team that authenticates his work. Reputable dealers and auction houses sell Banksy works only with certification.

Banksy has already sold art to benefit Ukraine. In March, he donated proceeds from the sale of a painting to the Okhmatdyt children’s hospital in Kyiv.

Mr. Dovhyi argued that the army’s needs during wartime justified the attempt to sell the painting of the woman in the bathrobe. “I wanted to capitalize as much as possible,” he said, “and all the money would go to humanitarian and military purposes.”

But the police turned up soon after he had cut the art from the wall, confiscating the slab of insulation and questioning Mr. Dovhyi and others in his group. Mr. Dovhyi said police told him the investigation is under an article of the criminal code that deals with property damage less severe than vandalism. He has not been charged.

Evelina Riabenko contributed reporting.

The flow of people leaving Ukraine will probably increase as winter deepens, even though the government has made a strenuous effort to repair the damage to energy infrastructure ravaged by Russian attacks, the head of the Norwegian Refugee Council humanitarian agency said on Tuesday.

The number of people leaving Ukraine has slowed sharply since the early weeks of the war — and many now cross back over the border — but the cold and the damage after two months of Moscow’s assault on water and power stations could soon reverse those gains, Jan Egeland, the organization’s secretary general, said.

“We haven’t yet seen the large exodus, but I fear that will come,” he said. “There has only been a fortnight of really cold weather.”

While many people who lose electrical or water service have regained it after a few days, Mr. Egeland said, that might not be sustainable much longer.

He described a conversation he had in late November with a woman in her 90s who was crossing into Ukrainian-controlled territory in the southern province of Zaporizhzhia. The woman, who arrived at the checkpoint with a daughter and grandchildren, said their house in the port city of Mariupol had been damaged during fighting in March. They were able to stay there for months — and had hoped to stay there through the winter — but it was just too cold to remain there any longer.

At the start of the invasion, a combination of Russian bombing, military advances and fear that Moscow would quickly overrun the government forced millions of people to flee to neighboring countries. In all, more than 7.8 million have registered as refugees, according to data from the United Nations’ refugee agency.

The number of returning refugees has picked up in recent months. About 20,000 people crossed back into Ukraine from Poland on Sunday, roughly the same number as headed out, according to Poland’s border agency.

But that balance is unlikely to last: Ukrainian government officials have said that people outside the country should not contemplate returning during the winter months given concerns about already stretched resources.

Many who fled their homes sought shelter in western Ukraine, particularly in rural areas less dependent on the national grid for energy. But Mr. Egeland said that pressure was likely to build in those areas, too, even as the government works to repair damage and set up centers where people can go for heat and basic services.

“I cannot see how people are able to sustain themselves through the long winter in overcrowded collective centers in central and western parts when there are more and more people being evacuated from the east,” he said.

The European Union condemned Iran’s military partnership with Russia on Monday as a gross violation of international law and announced new sanctions against eight Iranian individuals and entities over their role in supplying drones that Moscow has used to attack Ukrainian civilians and infrastructure.

Iranian-made drones have been used “indiscriminately by Russia against Ukrainian civilian population and infrastructure causing horrendous destruction and human suffering,” the European Council, the E.U.’s top decision-making body, said in a statement that broadly condemned Iran’s human rights record.

Russia has deployed Iranian-made drones in Ukraine since August, according to U.S. officials, to attack infrastructure as well as military targets. On Saturday, Russian forces used Iranian drones to knock out power to more than 1.5 million people in the port city of Odesa, far from the front lines.

The four individuals facing new E.U. sanctions include the commander in chief of the Iranian Air Force, the head of a weapons research and development unit of the Islamic Revolutionary Guard and a Revolutionary Guard officer who organized a drone demonstration for Russian forces in August.

The entities added to the sanctions list include what the European Council described as a “front company” involved in the development of drones used by Russia, as well as a company that the council says produced drone engines.

The bloc previously imposed sanctions on Iranians over the drone deliveries. In October, the European Union imposed restrictions on the chief of staff of Iran’s armed forces, the commander of the Revolutionary Guard’s air force and Shahed Aviation Industries, the company responsible for designing and developing the drones.

More than 1,350 individuals and entities have been subject to an asset freeze and travel ban by the bloc since 2014 over actions that “undermined Ukraine’s territorial integrity, sovereignty and independence.”

Iran’s military support for Russia has come under heightened international scrutiny in recent weeks, although the Kremlin has denied using the Iranian-made drones to attack civilians, and Tehran maintains that all drone deliveries took place before Russia’s invasion of Ukraine in February.

The Biden administration said on Friday that the two countries were forming a “full-fledged defense partnership” that was expected to intensify in the coming months, while British officials warned that Russia was seeking hundreds of ballistic weapons from Iran in exchange for “an unprecedented level of military and technical support.”

The European Union strongly advised Iran on Monday that any new weapon deliveries to Russia — in particular any movement toward delivering short-range ballistic weapons — would be considered “a serious escalation.”

KYIV, Ukraine — The pool has one dim light high on a wall. The windows are steamed up, because the ventilation is not working. Swimmers look like shadows under the water. Sometimes their fitness watches are the only light.

This gym usually has music to keep people motivated. But during the blackouts, as electricity blinks on and off after Russian missile attacks, it is very quiet. The only sound is of people talking and breathing and grunting as they work out.

“When I exercise, it distracts me from everything that is going on around,” said Yulia Kismonova, 42.

The attack on Ukraine’s electricity grid is intended by Russia to demoralize Ukrainians and force the country into negotiations, military analysts say. Keeping up routines, including staying in shape, is one form of Ukrainian defiance.

Ms. Kismonova just arrived at her gym, starting with the stepper to warm up. Every time she exercises, she also brings her daughters to swim in the pool.

Gyms in downtown Kyiv at first stopped operations when there were power cuts. But clients put pressure on managers, saying they needed to keep up their routines.

Many gyms now open their doors even when it is totally dark. If there is an air raid siren, trainers take children from the pool to the basement.

People use flashlights to find weights and navigate carefully from one machine to another. Once settled, they often switch off their lights, working out in darkness to save battery power.

Some gyms, however, have generators and can switch on the emergency lights when there are power cuts.

“Come on Pavlo, keep your legs higher,” said Polina Banakhova, teaching a 7-year-old boy in a pool with its emergency lights on.

“Children need their routines,” Ms. Banakhova said.

— Maria Varenikova