KYIV, Ukraine -- Ukraine accused Russia Thursday of bombing a theatre that was sheltering more than 1,000 civilians in the city of Mariupol, after US President Joe Biden branded Vladimir Putin a "war criminal." The latest assaults on civilians across Ukraine came as President Volodymyr Zelensky made a searing appeal for help to the US, which responded by pledging $1 billion in new weapons to fight Russia's invading army.

Officials across Ukraine are struggling to count the civilian dead -- with authorities saying 103 children have been killed since the invasion began -- who have been targeted in homes, hospitals, ambulances and food queues.

In the port city of Mariupol -- where more than 2,000 people have died so far -- a Russian bomb hit the Drama Theatre, which city council officials said had been housing over 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 out 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 that the building had been mined and blown up by members of Ukraine's far-right Azov Battalion.

In a statement, Human Rights Watch said that while it couldn't rule out the "possibility of a Ukrainian military target in the area of the theatre... we do know that the theatre had been housing at least 500 civilians."

"This raises serious concerns about what the intended target was in a city where civilians have already been under siege for days and telecommunications, power, water, and heating have been almost completely cut off," said Belkis Wille, senior crisis and conflict researcher at the rights watchdog.

So far the destruction that has marked other cities has been halted outside the capital Kyiv, which has been emptied of around half of its 3.5 million people.

But dull booms echoed across the capital's deserted streets Wednesday, with only an occasional vehicle passing through sandbagged checkpoints, and very few permits granted to break its latest curfew.

In an address to the US Congress, Zelensky invoked Pearl Harbor, the 9/11 attacks and Martin Luther King Jr as he showed lawmakers a video of the wrenching effect of three weeks of Russian attacks. (AFP)