The US embassy in Kyiv says it is “considering all available options to ensure accountability for any atrocity crimes in Ukraine”.
People are buried under rubble after a theatre in Mariupol – where hundreds of people are reported to have been sheltering – was bombed by Russian forces, local officials have said.
The city council said the number of casualties was not yet known, but Sky News has verified footage from the attack as showing the Donetsk Regional Drama Theatre.
A statement from Ukraine’s Foreign Ministry said: “The bomb strike demolished the central part of the theatre building, causing large numbers of people to be buried under the debris.
“The assessment of the exact number of persons affected is currently impossible due to ongoing shelling.
“By delivering a purposeful bomb attack to the place of mass gathering of civilians Russia has committed another war crime.”