More than 3.6 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 8% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in 28 days.

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

-ABC News' Zoe Magee

Russia claimed Wednesday that its forces carried out an airstrike destroying a weapons depot of the Ukrainian Armed Forces.

The Russian Ministry of Defense also alleged that troops have destroyed 430 aircraft, including drones, as well as more than 1,500 tanks and other combat armoured vehicles belonging to the Ukrainian Armed Forces since the "special military operation" began Feb. 24.

Negotiations between Ukraine and Russia are "very difficult" but "moving forward," Ukrainian President Volodymyr Zelenskyy said Wednesday.

"It's very difficult, sometimes confrontational," Zelenskyy said in an early morning address. "But step by step, we are moving forward."

Zelenskyy added that he is "grateful to all international mediators who are standing up for Ukraine."

Ukrainian President Volodymyr Zelenskyy will virtually address a NATO summit in Brussels on Thursday, his office confirmed to ABC News.

"We are still working on the format," his office said in a statement.

A NATO official also confirmed to ABC News that Zelenskyy will address the Extraordinary Summit of NATO Heads of State + Government and that they are still finalizing the details.

It is unclear if Zelenskyy will prerecord a statement or participate in the discussion.

-ABC News' Oleksii Pshemyskyi and Aicha El Hammar Castano

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