As Russian forces pressed their assault on Ukraine, world leaders called for an investigation of the Kremlin’s repeat attacks on civilian targets, including airstrikes on schools, hospitals and residential areas that led one official to lament that his city had never seen such “nightmarish, colossal losses.”

US Secretary of State Antony Blinken said Thursday that American officials were evaluating potential war crimes and that if the intentional targeting of civilians by Russia is confirmed, there will be “massive consequences.”

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In city after city, hospitals, schools and buildings where people sought safety from the bombardment have been attacked. Rescue workers searched for survivors in the ruins of a theatre that served as a shelter when it was blown apart by a Russian airstrike in the besieged city of Mariupol. And in Merefa, near the northeast city of Kharkiv, at least 21 people were killed when Russian artillery destroyed a school and a community centre, a local official said.

In the northern city of Chernihiv, dozens of bodies were brought to the morgue in just one day.

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The United Nations political chief, Undersecretary-General Rosemary DiCarlo, also called for an investigation into civilian casualties, reminding the UN Security Council on Thursday that international humanitarian law bans direct attacks on civilians.

She said many of the daily attacks battering Ukrainian cities “are reportedly indiscriminate” and involve the use of “explosive weapons with a wide impact area.” DiCarlo said the devastation in Mariupol and Kharkiv ”raises grave fears about the fate of millions of residents of Kyiv and other cities facing intensifying attacks.”

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In Mariupol, hundreds of civilians were reportedly taking shelter in a grand, columned theatre in the city’s centre when it was hit Wednesday by Russian forces. More than a day after the airstrike, there were no reports of deaths. With communications disrupted across the city and movement difficult because of shelling and other fighting, there were conflicting reports on whether anyone had emerged from the rubble.

Satellite imagery on Monday from Maxar Technologies showed huge white letters on the pavement in front of and behind the theatre spelling out “CHILDREN” in Russian — “DETI” — to alert warplanes to those inside.

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“We hope and we think that some people who stayed in the shelter under the theatre could survive,” Petro Andrushchenko, an official with the mayor’s office, told The Associated Press. He said the building had a relatively modern basement bomb shelter designed to withstand airstrikes. Other officials said earlier that some people had gotten out.

Video and photos provided by the Ukrainian military showed that the at least three-story building had been reduced to a roofless shell, with some exterior walls collapsed.