KYIV, Ukraine (AP) — About 300 people were killed in the Russian airstrike last week on a Mariupol theater that was being used as a shelter, Ukrainian authorities said Friday in what would make it the war’s deadliest known attack on civilians yet.

The bloodshed is certain to certain to fuel allegations Moscow has committed war crimes by killing civilians, whether deliberately or by indiscriminate fire.

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 grand, columned Mariupol Drama Theater, where hundreds of people were taking cover, the word “CHILDREN” printed in Russian in huge white letters on the ground outside to ward off aerial attack.

In announcing the death toll on its Telegram channel Friday, city officials 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 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.

The reported death toll came a day after 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 US 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, and President Vladimir Putin’s foreign minister, Sergey Lavrov, said the Western pressure amounts to “total war”.

“And the goals are not hidden,” he continued. “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, though it was not immediately clear if that included pro-Moscow separatist forces fighting in the east or others not part of the Defense Ministry, such as the National Guard. Earlier this week, NATO estimated that 7,000 to 15,000 Russian soldiers have been killed in four weeks of fighting.

Britain’s Ministry of Defense said Ukrainian forces have been counterattacking and have been able to reoccupy towns and defensive positions up to 35 kilometres (22 miles) east of Kyiv as Russian troops fall back on their overextended supply lines. In the south, logistical problems and Ukrainian resistance are slowing the Russians as they look to drive west toward the port of Odesa, the ministry said.

But the misery for civilians is growing more severe in Ukrainian 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 50 kilometres (30 miles) west of Kyiv, Russian troops who were in the village earlier in the week appeared to have been pushed out as part of a counteroffensive 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.

Tens of thousands of people have left Mariupol in the past week, most of them driving out in private cars through dozens of Russian checkpoints.

“Unfortunately, nothing remains of Mariupol,” said Evgeniy Sokyrko, who was among those waiting for an evacuation train in Zaporizhzhia, the closest urban centre to Mariupol and a way station for refugees. “In the last week, there have been explosions like I’ve never heard before.”

Oksana Abramova, 42, said she ached for those left behind in the city, who have been cut off from communication with the shelling of cell, radio and TV towers and lack the means to escape.

“Many people have no connection yet. All the time I think about how they are, where they are. Are still hiding, are they alive? Or maybe they are no longer there,” she said.

In Kyiv, ashes of the dead are piling up at the main crematorium in the capital because so many relatives have left, leaving urns unclaimed. And the northern city of Chernihiv is all but cut off.

Chernihiv lost its main road bridge over the Desna River to a Russian airstrike this week. Follow-up shelling then damaged a pedestrian bridge, trapping remaining inhabitants inside the city without power, water and heat, authorities said. More than half of Chernihiv’s prewar population of 285,000 is thought to have fled.