A Russian airstrike last week that blasted open a theater being used as a shelter in the suffering city of Mariupol killed about 300 people, Ukrainian authorities said, marking what could be the war’s deadliest known attack on civilians yet.

The death toll announced Friday fueled allegations that Moscow is committing war crimes by killing civilians, whether deliberately or with indiscriminate fire.

Russia, meanwhile, seemed to signal an important shift in its war objectives with an announcement that it plans to focus its forces on eastern Ukraine. U.S. officials said Russian troops appeared to have halted for now their ground offensive aimed at capturing the capital, Kyiv, and were concentrating more on gaining control of the Donbas region in the country’s southeast.

However, British defense officials reported Saturday that the Russian military continues to besiege a number of other major Ukrainian cities, including Kharkiv and Chernihiv in the north.

“It is likely Russia will continue to use its heavy firepower on urban areas as it looks to limit its own already considerable losses, at the cost of further civilian casualties,” the U.K. Ministry of Defense said in its latest intelligence briefing on the war.

Ukrainian President Volodymyr Zelenskyy again appealed to Russia to negotiate an end to the war, but pointedly said he would not give up any Ukrainian territory for the sake of peace.

“The territorial integrity of Ukraine should be guaranteed,” he said in a nightly video address to the nation. “That is, the conditions must be fair, for the Ukrainian people will not accept them otherwise.”

Mariupol, a strategically located port city in southeast Ukraine, has been under attack for almost the entire war. For days, the Mariupol government was unable to give a casualty count for the March 16 bombardment of the grand, columned Mariupol Drama Theater.

In an attempt to ward off an attack on the theater, where hundreds of people were sheltering, the word “CHILDREN” was printed in Russian in huge white letters on the ground outside.

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

U.S. President Joe Biden’s national security adviser, Jake Sullivan, said the theater 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” in the besieged port city.

The Ukrainian Parliament’s human rights commissioner said soon after the attack that more than 1,300 people had taken shelter in the theater, many of them because their homes were 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 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.”

Ukrainian officials continued to push for more military support. Zelenskyy’s chief of staff, Andriy Yermak, on Friday called for a lend-lease program, referring to U.S. policy of heavily supplying its World War II allies.

Ukraine needs real-time military intelligence and heavy weapons, Yermak said in an address.

While the Russians continue to pound the capital from the air, they appear to have gone into a “defensive crouch” outside Kyiv and are focused more on the Donbas, a senior U.S. defense official said, speaking on condition of anonymity to discuss the Pentagon’s assessment.

“They don’t show any signs of being willing to move on Kyiv from the ground,” the official said.

In comments that seemed to corroborate a change in Moscow’s military goals, Col.-Gen Sergei Rudskoi, deputy chief of the Russian general staff, said the main objective of the first stage of the operation — reducing Ukraine’s fighting capacity — has “generally been accomplished,” allowing Russian forces to focus on “the main goal, the liberation of Donbas.”

The Donbas is the largely Russian-speaking eastern part of the country where Russian-backed separatists have been fighting Ukrainian forces since 2014 and where many residents desire close ties to Moscow.

Its coal-mining and industrial Donetsk and Luhansk regions are recognized by Russia as independent. Mariupol is also in the Donbas, although outside of the territories controlled by separatists since 2014.

Britain’s Ministry of Defense said Ukrainian forces have been counterattacking and have been able to reoccupy towns and defensive positions up to 35 kilometers (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.

In fact, the Russians are no longer in full control of Kherson, the first major city to fall to Moscow’s forces, the senior U.S. defense official said. The official said the southern city is being contested by the Ukrainians in heavy fighting. The Kremlin denied it had lost full control.

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 the separatists 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.

For civilians, the misery 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 kilometers (30 miles) west of Kyiv, Russian troops who occupied it earlier in the week appeared to have been pushed out as part of a counteroffensive by Ukrainian forces.

The village church’s tower was damaged by a blast, and houses on the main crossroads lay in ruins.

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

In Kyiv, ashes of the dead are piling up at the main crematorium because so many relatives have left, leaving urns unclaimed. And the northern city of Chernihiv is all but cut off after Russian forces destroyed bridges, leaving people without power, water and heat, authorities said.

For the vulnerable — the elderly, children and others unable to join millions fleeing westward — food shortages are mounting in a country once known as the breadbasket for the world.

In relentlessly shelled Kharkiv, hundreds of panicked people took shelter in the subway. A hospital emergency room filled with wounded soldiers and civilians.

Mostly elderly women lined up stoically to collect food and other urgent supplies this week, as explosions thudded in the distance. Fidgeting with anticipation, a young girl watched as a volunteer’s knife cut through a giant slab of cheese, carving out thick slices, one for each hungry person.

As for Mariupol, “nothing remains of Mariupol,” said Evgeniy Sokyrko, who was among those waiting for an evacuation train in Zaporizhzhia, a way station for refugees from the shattered port city. “In the last week, there have been explosions like I’ve never heard before.”

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Rosa reported from Kharkiv, Ukraine. Associated Press writer Robert Burns in Washington and journalists around the world contributed to this report.