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Russia has bombed a theatre that was sheltering more than 1,000 civilians in Mariupol, Ukraine said on Thursday.
It comes after US President Joe Biden called Russian President Vladimir Putin a "war criminal".
As more assaults against civilians took place across Ukraine, President Volodymyr Zelenskyy's appeal for help was met by a US pledge of $1 billion in new weapons to fight Russia's invading army.
"The bomb shelter held," mayoral adviser Petro Andrushchenko told Reuters. "Now the rubble is being cleared. There are survivors. We don’t know about the (number of ) victims yet."
He said rescue work was underway to reach survivors and establish the number of casualties, which was still unknown.
Ukraine officials are struggling to count the civilian dead, with authorities saying 103 children have been killed since Russia's invasion of Ukraine began.
Civilians have been targeted in their homes and in hospitals, ambulances and food queues.
Russian foreign ministry spokeswoman Maria Zakharova said on Thursday that the allegation that Russia had bombed the theatre was a "lie".
She repeated Kremlin denials that Russian forces have targeted civilian areas.
"Russia's armed forces don't bomb towns and cities," she said at a briefing.
In the port city of Mariupol, where more than 2,000 people have died so far, a Russian bomb hit a drama theatre, which city council officials said had been housing more than 1,000 people.
"The only word to describe what has happened today is genocide, genocide of our nation, our Ukrainian people," the city's mayor Vadim Boychenko said in a video message on Telegram.
"We have difficulty understanding all of this, we refuse to believe, we want to close our eyes and forget the nightmare that happened today," he said.
Satellite images of the theatre on March 14 shared by private satellite company Maxar showed the words "children" clearly etched in the ground in Russian on either side of the building.
Officials posted a photo of the building, whose middle part was completely destroyed, with thick white smoke rising from the rubble after they said a bomb was dropped from an airplane.
"It is impossible to find words to describe the level of cynicism and cruelty, with which Russian invaders are destroying peaceful residents of a Ukrainian city by the sea," an official statement read.
Russia's defence ministry denied it had targeted the theatre, instead claiming the building had been mined and blown up by members of Ukraine's far-right Azov Battalion.
The US president also stepped up his condemnation of the Russian leader, describing him as a "war criminal".