During a virtual bilat with Irish Taoiseach Micheál Martin on Thursday, President Joe Biden said the world is "united" as Russia's invasion in Ukraine continues.

“We have to be united and we certainly are,” Biden said. “But Putin’s brutality and what he's doing, and his troops are doing in Ukraine, is just inhumane."

The Taoiseach told Biden, “I share with you our horror at the barbaric attack on the civilians,” and said Biden’s leadership through this has been “firm,” “determined" and “strong.”

Biden commended Ireland for taking in Ukrainian refugees, saying it "speaks so loudly about your principles."

-ABC News' Justin Ryan Gomez

President Joe Biden will speak to Chinese President Xi Jinping on Friday about “Russia’s war against Ukraine,” among other topics, White House press secretary Jen Psaki said in a statement.

It’s the first time the two will speak since Russia’s invasion began and it follows National Security Advisor Jake Sullivan’s seven-hour face-to-face meeting in Rome with his Chinese counterpart earlier this week.

The U.S. has been ramping up its warnings to China over concerns that it could assist Moscow with military equipment and other aid.

-ABC News' Justin Ryan Gomez

Russia’s invasion of Ukraine has “largely stalled on all fronts,” the U.K. Ministry of Defence said on Thursday.

“Russian forces have made minimal progress on land, sea or air in recent days and they continue to suffer heavy losses,” the Ministry said in an update posted to Twitter.

The Ukrainian resistance “remains staunch and well-coordinated,” the update said.

“The vast majority of Ukrainian territory, including all major cities, remain in Ukrainian hands,” the Ministry said.

A Ukrainian official claimed Wednesday that Russian airstrikes destroyed a theater in the besieged city of Mariupol where civilians were taking shelter.

The number of victims from the bombing of the Donetsk Regional Theatre of Drama "is impossible to count," Pavlo Kyrylenko, head of the Donetsk Region administration, said in a Facebook post.

Ukrainian President Volodymyr Zelenskyy said during an address tonight that hundreds of people were hiding in the theater and that the death toll is still unknown.

"Russia is killing civilians!" Kyrylenko said, adding that it is also "impossible to determine" the number of victims in Mariupol since the start of the invasion.

The city has been burying its dead in a mass grave on the outskirts of Mariupol as it endures heavy shelling.

-ABC News' Christine Theodorou

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