"You already know that they dropped bombs on the hospital, aerial bombs destroyed the hospital, and that is a sign of a war crime, because the number of wounded before that was 170, and now it is over 600," the mayor, Vadym Boichenko, said on Ukrainian television.
The Azovstal plant was heavily bombed on Wednesday night, according to multiple accounts.
The Azov said the Russians hit an improvised underground hospital and its surgery room, killing an unspecified number of people and wounding others there.
Mariupol authorities are sounding the alarm about unsanitary conditions in the ravaged port city that they say pose a "deadly danger" to its remaining residents.
"Deadly epidemics may break out in the city due to the lack of centralised water supply and sewers," the city council said.
The lives of 100,000 people who still remain in Mariupol, out of 450,000 pre-war residents, may be in danger – pointing to diseases like cholera and dysentery.