Russia-Ukraine updates: US sanctions Russian military shipbuilder, diamond miner

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

In his latest national address, Ukrainian President Volodymyr Zelenskyy said negotiations with Russia are "ongoing."

"My priorities in the negotiations are absolutely clear: the end of the war, security guarantees, sovereignty, restoration of territorial integrity, real guarantees for our country, real protection for our country," he said in a speech that aired tonight.

Zelenskyy said he addressed both the U.S. and all the relevant states in regard to creating a new union he called U-24. He said that the new alliance will ensure that aggressors receive a coordinated response from the world.

"We can no longer trust the existing institutions. We cannot expect bureaucrats in international organizations to change so quickly," he said. "Therefore, we must look for new guarantees. Create new tools. Take those who have courage and do what justice requires."

The United Nations Security Council will hold an emergency meeting on Thursday to address the humanitarian situation in Ukraine.

The U.S., Albania, U.K., France, Ireland and Norway requested the meeting, according to the Norway U.N. The countries have asked for briefings by the U.N.'s Department of Political and Peacebuilding Affairs, the U.N.'s refugee agency and the World Health Organization.

More thank 3 million refugees have alreay fled Ukraine since the invasion began on Feb. 24, according to the U.N.'s refugee agency.

-ABC News' Will Gretsky