There was no immediate response to the allegations from Ukraine. Reuters was not able to independently verify the report.
The alleged strike hit a hospital in the Russian-held settlement of Novoaidar and was carried out using a U.S.-supplied HIMARS rocket launch system, the Russian defence ministry said in a statement.
"A deliberate missile strike against a known functioning civilian medical facility is without doubt a serious war crime by the Kyiv regime," the defence ministry said.
"All those involved in the planning and execution of this crime will be found and held accountable."
Civilian and military medics had been working in the hospital for many months treating local people and soldiers, it said.
Ukraine and its Western allies have accused Russian forces of frequent war crimes in the conflict in which thousands of civilians have been killed and cities and towns pounded by artillery and air strikes. Russia denies targetting civilians.
(Reporting by ReutersEditing by Andrew Osborn and Angus MacSwan)