Ukrainian President Volodymr Zelenskiy accused Russia on Wednesday of carrying out an airstrike that severely damaged a children's hospital in the southern port city of Mariupol.
Children were among people "under the wreckage," he said, without giving details of casualties.
Mariupol's city council said the hospital had been destroyed. It said it did not know any casualty figures but added: "The destruction is colossal."
The reports could not immediately be verified by Reuters. Russia has denied targeting civilians in its assault on Ukraine.
"Direct strike of Russian troops at the maternity hospital. People, children are under the wreckage," Zelenskiy said on Twitter.
"Atrocity! How much longer will the world be an accomplice ignoring terror? Close the sky right now! Stop the killings! You have power but you seem to be losing humanity."
The Ukrainian Foreign Ministry carried video footage on Twitter of what appeared to be the badly damaged hospital under the headline: "Today, Russia shelled a Childrenʼs hospital and a Maternity hospital in Mariupol."
The footage showed holes where windows should have been in what appeared to be a badly damaged three-storey building at the hospital, and huge piles of rubble, some of it smouldering.
Ukraine had earlier on Wednesday accused Russia of breaking a ceasefire to prevent the evacuation of hundreds of thousands of civilians trapped in Mariupol, where the Red Cross has described conditions as "apocalyptic".
Russia said it would hold fire to let civilians flee besieged cities, but efforts to evacuate Mariupol appeared to have failed again, as have several previous attempts since Saturday.