At least four people were killed and many more wounded when a shell hit a hospital in the rebel-held city of Donetsk in eastern Ukraine.
Violence has surged in recent weeks, and the pro-Russian rebels are trying to encircle the key town of Debaltseve, northeast of Donetsk, BBC reported.
EU foreign policy chief Federica Mogherini appealed for a truce to allow civilians to leave the area.
According to Amnesty International, 7,000 people remain in Debaltseve, a major road and rail hub, out of an original population of 25,000 a few days ago.
The separatists have already captured Donetsk airport from government forces in the past few weeks.
The hospital in the southwestern Kirovskiy area of Donetsk was being used as an outpatient clinic at the time of the attack and many of the injured were patients and staff.
Mortar rounds have regularly hit residential areas of the two main rebel-held cities of Donetsk and Lugansk and the separatists have blamed the government for the attacks.
Ukrainian officials responded by insisting that soldiers did not fire on civilian areas and did not use cluster munitions.
More than 5,000 people have been killed in the conflict, which began in the east last April, when separatists seized government buildings after the peninsula of Crimea acceded to Russia.
US Secretary of State John Kerry is due to visit Kiev Thursday.
