Russian forces dropped a powerful bomb on a theater in the encircled Ukrainian port city of Mariupol where hundreds of civilians were sheltering on Wednesday, Ukraine's foreign ministry said.

The ministry said many people were trapped in the theater and accused Russia of committing a war crime. It said the number of casualties was not yet known. Reuters could not independently verify the information.

Russia denies targeting civilians. In Moscow, the defense ministry said its forces had not struck the building and instead accused the Azov Battalion, a far-right Ukrainian militia, of blowing it up, RIA news agency said.

It did not give evidence to back up the claim. Russia had previously accused the battalion of preventing civilians from leaving the city, which has come under heavy bombardment.

Maxar Technologies, a private U.S. company, distributed satellite imagery that it said was collected on March 14 and showed the word "children" in large Russian script painted on the ground outside the red-roofed Mariupol Drama Theatre.

Maxar said it would distribute new images of the theater as soon as it has them.