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.”
A satellite image taken by Maxar Technologies shows the theatre on 14 March before the attack.
Spelt out on the ground is the Russian word for “children”, which shows how it was clearly identified for days that people were sheltering in it.
However, the RIA news agency, reported Russia’s Defence Ministry has denied it carried out the attack, instead
accusing the Azov Battalion, a far-right Ukrainian militia, of blowing it up.
It did not give evidence to back up the claim.
Earlier, Russian forces shot and killed 10 people queuing for bread in Chernihiv, northern Ukraine, the US embassy in Kyiv has said.