Russian forces dropped a bomb on a theatre where civilians were being sheltered in the besieged city of Mariupol, local officials say.
Ukrainian officials have accused Russia of purposely destroying a theatre in Mariupol on Wednesday where about 1,200 people were sheltering.
"The bomb strike demolished the central part of the theatre building, casuing large numbers of people to be buried under the debris," Ukraine's Foreign Ministry said in a statement.
Assessing casualty levels was currently impossible "due to the ongoing shelling".
The foreign minister has tweeted a before and after of the theatre.
Mariupol mayor Vadym Boichenko called the attack a 'horrifying tragedy'.
"People were hiding thereā¦ some said they were lucky to survive, but unfortunately not all were lucky," he said, indicating there were deaths.
"The only word to describe what has happened today is genocide... genocide of our nation, our Ukrainian people. But I am confident that the day will come when our beautiful city of Mariupol will rise out of the ruins again."
Russia's defence ministry said the theatre was destroyed by Ukraine's nationalist Azov battalion, and claimed civilians had been held 'hostage' at the site.
Local authorities say at least 2,400 people have been killed in Mariupol since the start of the war, although they acknowledge this is likely to be an underestimate. Many of the dead are being buried in mass graves.
An estimated 300,000 residents are trapped inside the city, where running water, electricity and gas have been cut off. Food and water supplies are running low, as Russian troops have not allowed the delivery of humanitarian aid.
"Russian troops have cynically and deliberately destroyed the Drama Theater in the heart of Mariupol. The plane has dropped a bomb on a building in which hundreds of peaceful residents were hiding," the Mariupol City Council reported on its Telegram channel.
"It is still impossible to estimate the scale of this horrible and inhumane act because residential areas of the city continue to be shelled. It is known that after the shelling, the central part of the Drama Theater was destroyed and the entrance to the air-raid shelter in the building was destroyed," the City Hall added.
Hours after news of the destruction emerged, the Russian defence ministry denied it had carried out an air strike against the theatre, the RIA news agency reported.
About 1,500 cars had managed to flee Mariupol on Wednesday, according to Mr Orlov, the deputy mayor. But, he said, an attack by Russia on the convoy left at least five wounded, including a child.
