More than 4.1 million people have been forced to flee Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the United Nations Refugee Agency.

The tally from the U.N. High Commissioner for Refugees (UNHCR) amounts to just over 9.2% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in 36 days.

More than half of the refugees crossed into neighboring Poland, UNHCR figures show.

-ABC News' Zoe Magee

Russia has accused Ukraine of carrying out airstrikes on the Russian city of Belgorod early Friday.

Belgorod Oblast Gov. Vyacheslav Gladkov said in a statement via Telegram that two low-flying Ukrainian helicopters entered Russian airspace and fired on an oil depot in Belgorod city, setting the building ablaze. Ukraine has yet to comment on the claim.

The depot run by Russian energy giant Roseneft is located about 21 miles north of the border with Ukraine. Two employees were injured but are expected to survive, while all other staff have been safely evacuated from the building, according to Gladkov.

Russia's Defense Ministry claimed the oil facility only supplied fuel to civilian vehicles, and has no relation to Russia's armed forces.

"Two Ukrainian Mi-24 helicopters entered the Russian Federation's airspace at an extremely low altitude at about 5 o'clock in the morning Moscow time on April 1. The Ukrainian helicopters delivered a missile strike upon a civilian oil storage terminal located on the outskirts of Belgorod. A number of tanks were damaged and caught fire after being hit with missiles," Igor Konashenkov, a spokesperson for Russia's Defense Ministry, said at a press briefing.

He added: "I'd like to stress that this facility is used to supply fuel only to civilian transport vehicles. The oil terminal has no relation to the Russian Armed Forces."

Security camera footage circulating online and verified by ABC News shows an attack on an oil depot in Belgorod. In the video, two airstrikes can be seen in the distance, with a helicopter flying nearby.

Another verified video circulating online shows oil tanks on fire and a massive cloud of smoke billowing from the depot.

Russian news agency Interfax reported that at least two businesses in the village of Severny, just north of Belgorod, were also damaged by an early morning airstrike.

It remains unclear who is responsible for the attacks.

Belgorod, a city of more than 300,000, is about 50 miles north of the Ukrainian city of Kharkiv, which Russian forces have shelled heavily in recent weeks.

-ABC News' Victoria Beaule

An estimated 100,000 civilians remain trapped in Ukraine's besieged port city of Mariupol despite repeated efforts by Ukrainian officials to evacuate them, according to Petro Andryushenko, adviser to Mariupol's mayor.

Andryushenko told ABC News on Friday morning that Russia has not confirmed any humanitarian corridors leading out of Mariupol since announcing a localized cease-fire on Thursday to allow civilians to be evacuated.

A convoy of 45 evacuation buses that were sent to Mariupol have yet to reach the southeastern port city because it remains under Russian lockdown, according to Andryushenko, who noted that some people managed to escape by foot or in their own cars.

-ABC News' Oleksii Pshemysky

Russian troops that have begun to withdraw from the ground effort against Ukraine's capital of Kyiv seem to be heading north to Belarus to regroup before rejoining the fight, the Pentagon said Thursday.

"The best assessment we have – and it's an assessment at this early stage – is that they're going to be repositioned probably into Belarus to be refit and resupplied, and used elsewhere in Ukraine," Pentagon press secretary John Kirby told reporters.

It's not clear where they might go, but the Donbas region is one candidate, Kirby said.

Roughly 20% of the Russian forces that were designated to move on Kyiv are now repositioning, several U.S. officials said.

Kirby said Russian forces that are apparently leaving the Chernobyl nuclear power plant also seem to be heading toward Belarus, though noted that "indications are not completely clear at this time."

The Pentagon assesses these troops are leaving to "refit and resupply," and not due to a health hazard or other crisis at Chernobyl, Kirby said.

-ABC News' Matthew Seyler

All Russian troops have left the Ukrainian cities of Kyiv and Chernihiv, withdrawing north toward the borders of Belarus and Russia to consolidate before likely redeploying to the Donbas region in eastern Ukraine, a senior U.S. defense official told reporters Wednesday.

But even with the Russians gone, the territory remains treacherous.

"There are some indications that they left behind mines and things like that, so the Ukrainians are being somewhat careful in some areas north of Kyiv as they begin to clear the ground and clear the territory and re-occupy it," the official said.

While the U.S. hasn't yet seen these troops redeploy elsewhere in Ukraine, it'll likely happen soon, according to the official. Ukrainian forces are preparing for a major fight in Donbas, the official said.

The official also said the Pentagon is "monitoring" an apparent nitric acid explosion in Ukraine's Luhansk region, which Russia blamed on Ukraine.

"We've seen the Russians claim that this was a Ukrainian attack on this. We do not believe that is true," the official said. "We do believe that the Russians are responsible, but exactly what they used when they did it, why they did it, what the damage is, we just don't have that level of detail," the official said.

The official also noted that a small number of Ukrainians currently in the U.S. for "professional military education" were pulled aside for a couple days of training on Switchblade drones, which the U.S. is sending overseas as part of its military aid, according to the official.

"Although it's not a very difficult system to operate, we took advantage of having them in the country to give them some rudimentary training on that," the official said.

-ABC News' Matt Seyler