The United Nations on Tuesday condemned a recent wave of airstrikes on medical centers in rebel-held parts of Syria, including one that put a hospital serving 50,000 people out of action.

The Syrian government, which is backed by Russian air power in its almost seven-year-long war with rebels, says it only targets militants and has repeatedly denied striking civilian facilities such as hospitals.

"I am appalled by the ongoing attacks on hospitals and other medical facilities in northwestern Syria, depriving hundreds of thousands of people of their basic right to health," said Panos Moumtzis, U.N. regional coordinator for the Syria crisis.