Britain on Thursday accused Russia of a “war crime” for an attack the previous day on a children’s and maternity hospital in the besieged Ukrainian city of Mariupol. Ukrainian officials have said that at least three people, including a young girl, were killed in the strike, which Ukrainian President Volodymyr Zelensky labelled an “atrocity” soon after it happened.

U.K. armed forces minister James Heappey said Thursday that regardless of whether it was “indiscriminate” fire by Russia into a built-up area or a deliberate targeting of a health facility, “it is a war crime.”

U.S. Vice President Kamala Harris, speaking alongside Poland’s president Thursday in Warsaw, noted the attack on the hospital and said the world was witnessing “atrocities of unimaginable proportions in Ukraine.” 

“We stand with the people of Ukraine,” Harris said after reaffirming America’s “ironclad” commitment to defend NATO nations.  

Asked later if she believed Russia’s actions should be investigated as possible war crimes, Harris said “when it comes to crimes and violations of international norms” the U.S. was “clear that any intentional attack on innocent victims is a violation.”

She said the United Nations had an established process to determine whether war crimes had been committed, and “absolutely there should be an investigation. The eyes of the world are on this war and what Russia has done.”  

Speaking after talks with his Ukrainian counterpart Dmytro Kuleba on Thursday in Turkey, Russian Foreign Minister Sergey Lavrov dismissed the claims of a Russian attack on a functioning hospital as lies and propaganda. 

“It is not the first time we have seen pathetic outcries concerning the so-called atrocities” by Russia, Lavrov said, claiming the hospital was being used as a base by an “ultra-radical” Ukrainian battalion. He said Russia had submitted data to the United Nations “days ago” to prove its claim, and accused foreign media of manipulating information on the strike.

Lavrov claimed no patients or staff had been at the facility, which he insisted had “long ago become a base for extremists.”

CBS News senior foreign correspondent Charlie D’Agata said the massive airstrike on the hospital in Mariupol shattered a fragile cease-fire in the southern port city of Mariupol late Wednesday afternoon amid efforts to evacuate civilians from the besieged city.

Emergency teams and soldiers scrambled to evacuate the wounded, including pregnant women, from the hospital. The blast destroyed the complex inside and out, and the size and depth of the crater and the surrounding debris were clear evidence of its ferocity, D’Agata said. 

Mariupol has come under heavy Russian bombardment for days, cutting off power and water to more than 400,000 people trapped in the city. Ukrainian officials say at least 1,200 civilians have been killed there since the war began, and images have shown city workers placing bodies into a mass grave.  

Ukrainian officials said at least seven more civilians were killed in further Russian artillery attacks overnight, and the city council said on Thursday that the rocket fire continued, hitting more civilian infrastructure.

“Bombs are hitting houses,” the council said in a social media post.

A humanitarian convoy trying to reach the city was forced to turn back on Thursday because of ongoing fighting, Ukrainian Deputy Prime Minister Iryna Vereshchuk said.

After his discussion with Lavrov on Thursday, Ukraine’s Kuleba said he was prepared to meet his counterpart again to “continue engagement” aimed at first establishing a cease-fire and humanitarian corridor for Mariupol, saying the city was at the epicenter of the humanitarian crisis in his country.