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

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

Ukraine’s military said it is trying to launch counter-attacks in northern areas on the edge of Kyiv, seeking to push Russian forces back from the towns at the gates of the capital where they've been bogged down for two weeks.

The sounds of intense shelling and fighting could be heard from the north of Kyiv the last three days. Battles have been raging in the towns of Irpin, Bucha and Hostomel, just a few miles from the city limits and from where thousands of civilians have been fleeing.

"The situation remains difficult, especially in the south and east [of Ukraine]. But more and more often our defenders are moving into counterattacks in various parts of the front: from Kyiv and Mykolaiv regions to the Luhansk region,” Ukrainian officials said in a statement Wednesday, referring to regions in southern and eastern Ukraine.

Authorities have imposed a full curfew from Tuesday evening to Thursday morning, locking down the capital and forbidding people from going outside. Plumes of smoke could be seen rising from the direction of the northern areas and the popping sound of small arms fire heard occasionally throughout the day Wednesday.

-ABC News' Patrick Reevell