About 300 people were killed in the Russian air strike on a Mariupol theatre that was being used as a shelter, Ukrainian authorities said, in what would make it the war’s deadliest attack on civilians.

The bloodshed fuelled allegations that Moscow is committing war crimes by killing civilians, whether deliberately or by indiscriminate fire, with a Nato official saying Russian President Vladimir Putin’s war is “unprovoked, illogical and also barbarian”.

For days, the government in the besieged and ruined port city was unable to give a casualty count for the March 16 bombardment of the Mariupol Drama Theatre, where hundreds of people were said to be taking cover.

The word “Children” was printed in Russian in huge white letters on the ground outside in a bid to ward off an aerial attack.

In announcing the death toll on its Telegram channel on Friday, the city government cited eyewitnesses. But it was not immediately clear how witnesses arrived at the figure or whether emergency workers had finished excavating the ruins.

US President Joe Biden’s national security adviser Jake Sullivan said on Friday the theatre bombing was an “absolute shock, particularly given the fact that it was so clearly a civilian target”.

He said it showed “a brazen disregard for the lives of innocent people”.

The scale of devastation in Mariupol, where bodies have been left unburied amid bomb craters and hollowed-out buildings, has made information difficult to obtain.

But soon after the attack, the Ukrainian Parliament’s human rights commissioner said more than 1,300 people had taken shelter in the theatre, many of them because their homes had been destroyed. The building had a basement bomb shelter and some survivors did emerge from the rubble after the attack.

“This is a barbaric war, and according to international conventions, deliberate attacks on civilians are war crimes,” said Mircea Geoana, Nato’s deputy-secretary general.

He said Mr Putin’s efforts to break Ukraine’s will to resist are having the opposite effect.

“What he’s getting in response is an even more determined Ukrainian army and an ever more united West in supporting Ukraine,” he said.

On Thursday, Mr Biden and allied leaders promised more military aid for Ukraine is on the way. But Nato has rejected Ukrainian President Volodymyr Zelenskyy’s urgent pleas to supply warplanes or establish a no-fly zone over his country for fear of getting into a war with Russia.

The U.S. and the European Union on Friday did announce a move to further squeeze Russia economically: a partnership to reduce Europe’s reliance on Russian energy and dry up the billions of dollars the Kremlin gets from the sale of fuel.

Moscow is bristling at the tightening noose of sanctions around Russia’s economy. Russian Foreign Minister Sergey Lavrov said the Western pressure amounts to “total war”.

“And the goals are not hidden,” he said. “They are declared publicly — to destroy, break, annihilate, strangle the Russian economy and Russia on the whole.”

The Russian military said 1,351 of its soldiers have died in Ukraine and 3,825 have been wounded.

Earlier this week, Nato estimated that 7,000 to 15,000 Russian soldiers have been killed in four weeks of fighting.

Ukrainian forces have been counterattacking and have been able to reoccupy towns and defensive positions up to 22 miles east of Kyiv as Russian troops fall back on their overextended supply lines.

But the misery for civilians is growing more severe in towns and cities which increasingly resemble the ruins that Russian forces left behind in their campaigns in Syria and Chechnya.

In the village of Yasnohorodka, some 30 miles west of Kyiv, Russian troops who were there earlier in the week appeared to have been pushed out as part of a counter-offensive by Ukrainian forces.

The tower of the village church was damaged by a blast, and houses on the main crossroads lay in ruins. Loud explosions and bursts of gunfire could be heard.

“You can see for yourself what happened here. People were killed here. Our soldiers were killed here. There was fighting,” said Yasnohorodka resident Valeriy Puzakov.