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

About 300 people were killed last week in a Russian airstrike on a drama theater-turned-bomb shelter in the besieged Ukrainian city of Mariupol, the city's government said Friday, citing eyewitnesses.

"We didn't want to believe in this horror," the Mariupol City Council. said in a statement. "But the words of those who were inside the building at the moment of this terrorist act say the opposite."

As many as 1,500 civilians had been taking refuge in the grand, columned Donetsk Regional Theatre of Drama in central Mariupol when it was struck on March 16, according to the Ukrainian government. Satellite images showed huge white letters on the pavement in front of and behind the building spelling out "CHILDREN" in Russian -- "DETI" -- to alert warplanes to those inside.

Video circulating online and verified by ABC News shows the immediate aftermath of the strike on the theater. People covered in dust are seen trying to make their way out of the theater, walking down from the first floor staircase in an area of the building that was still standing at the time.

Since invading Ukraine on Feb. 24, Russian forces have been relentlessly bombarding Mariupol, destroying homes and leaving thousands of residents trapped. Ukraine has defied Russia's ultimatum for its troops to lay down arms and surrender the strategic southeastern port city of 430,000.

-ABC News Patrick Reevell

Russia claimed Friday that its forces had captured five more localities in Ukraine.

"The grouping of troops of the Russian Armed Forces advanced another 4 kilometers overnight and captured Batmanka, Mikhailovka, Krasny Partizan, Stavki and Troitskoe," the Russian Ministry of Defense said in a statement.

Ukraine did not immediately comment on the claim.

U.S. President Joe Biden and European Commission President Ursula von der Leyen announced Friday a joint task force to "reduce Europe's dependence on Russian fossil fuels and strengthen European energy security," amid Russia's invasion of Ukraine.

Named the "Task Force for energy security," the group will be chaired by one representative from the White House and one representative from the European Commission. They will work to ensure energy security for Ukraine and the European Union ahead of the next two winters by focusing on two main goals -- diversifying liquefied natural gas supplies and reducing demand for natural gas, according to a fact sheet from the White House.

As part of the agreement, the United States will work with international partners to put more liquefied natural gas on the EU market, pledging to make at least 15 billion cubic meters available in 2022, with increases expected going forward.

The White House stressed that the task force would also work with an eye towards clean energy, looking to reduce greenhouse gas intensity of all new liquefied natural gas infrastructure as well as demand for liquefied natural gas by "accelerating market deployment of clean energy measures." Those measures include expediting planning of clean energy projects, like wind and solar power, and using smart thermostats and heat pumps in homes.

-ABC News' Molly Nagle

U.S. President Joe Biden's final meeting in Brussels on Thursday was with the European Council.

"They reviewed their ongoing efforts to impose economic costs on Russia and Belarus, as well as their readiness to adopt additional measures and to stop any attempts to circumvent sanctions," the White House said in a statement.

The leaders said they willl continue "providing humanitarian assistance, including to neighboring countries hosting refugees, and underscored the need for Russia to guarantee humanitarian access to those affected by or fleeing the violence," according to the White House.

They also "discussed EU-U.S. cooperation to reduce dependence on Russian fossil fuels, accelerate the transition to clean energy, as well as the need to respond to evolving food security needs worldwide," the White House said.

-ABC News' Molly Nagle