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

The besieged Ukrainian city of Mariupol seems to be facing particular brutalization by Russian forces, including "some of the most egregious accounts of what could constitute war crimes," U.S. Department of State spokesperson Ned Price said Tuesday.

The southeastern coastal city's strategic location could be a factor, Price told reporters during a briefing. There may also be a "particular vengeance against this population," he added, with Putin "having been under the impression -- whether he was misinformed or just unwitting of reality -- that his forces would not be greeted as anything other than the aggressors that they are."

"Every day when it comes to potential atrocities, we are seeing, we are all witnessing additional evidence that the Russians are in fact committing atrocities, committing acts that may go above that threshold to include potential war crimes," Price said.

Price said the U.S. still hasn't confirmed Russia's claims that it has evacuated thousands of Ukrainian civilians from the city to Russian territory.

-ABC News' Conor Finnegan

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