The Ukrainian Naval Forces said Thursday that it has sunk the Russian ship Orsk in the Sea of Asov near the southeastern Ukrainian port city of Berdyansk.

The Ukrainian Naval Forces released a video, verified by ABC News, showing a large ship burning in the Russian-occupied port of Berdyansk, with flames and thick smoke billowing into the sky. The Orsk, a Russian Navy amphibious landing ship that was used to ferry Russian troops to Ukraine, was docked in the port.

Russia did not immediately comment on the claim.

-ABC News' Fergal Gallagher

More than 3.67 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 8.3% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in 29 days.

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

-ABC News' Zoe Magee

NATO leaders, including U.S. President Joe Biden, posed for a photo at the alliance's headquarters in Brussels on Thursday ahead of an emergency summit, as the Russian invasion of Ukraine grants into a second month.

Biden stood in the front row in between NATO Secretary-General Jens Stoltenberg and U.K. Prime Minister Boris Johnson.

Biden and Stoltenberg were the last of the leaders to arrive for the photo-op. As they walked in the room, Biden ignored a question from a reporter about what his message is to Russian President Vladimir Putin. Instead, Biden went to shake hands with Johnson and then greeted French President Emmanuel Macron and Italian Prime Minister Mario Draghi

-ABC News' Justin Gomez

U.S. President Joe Biden arrived in Brussels on Thursday morning ahead of an emergency NATO summit to discuss the Western defense alliance's response to Russia's invasion of Ukraine.

The European diplomatic capital is also hosting a gathering of the Group of Seven industrialized nations and a summit of the 27 members of the European Union on Thursday. Biden is scheduled to attend all three meetings and hold a press conference at the end of the day.

Upon his arrival at NATO headquarters, Biden was greeted by NATO Secretary-General Jens Stoltenberg. The two leaders stood and spoke for a few moments, though their conversation was inaudible. Biden and Stoltenberg then walked into the building and down the hallway, where they did not stop to speak to reporters who asked whether Russia's potential use of chemical weapons in Ukraine are a red line that would trigger a response from NATO.

Biden and Stolenberg will now meet privately before taking a photo with other NATO leaders.

Earlier Thursday, U.K. Prime Minister Boris Johnson faced the same question from reporters when he arrived at NATO headquarters.

"Look, I think that the reality is that President Putin has already crossed a red line in barbarism and it's now up to NATO to consider together the appalling crisis in Ukraine, the appalling suffering of the people of Ukraine," Johnson replied. "And see what more we can do to help the people of Ukraine protect themselves. See what more we can do to tighten the economic vice around the Putin regime."

-ABC News' Justin Gomez, Molly Nagle and Cecilia Vega

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