Hospitals have been ripped apart by airstrikes. Nearly 50,000 people have fled their homes, and tens of thousands lack running water. Civilians are being laid out in body bags on hospital floors after shells struck their neighborhoods.

Scenes from the bloodiest days of Syria’s civil war, which had lain largely dormant for several years, are now repeating themselves in the country’s northwest as pro-government forces try to beat back a surprise rebel offensive, according to aid workers, a war monitor and the United Nations, who warned of a rapidly worsening humanitarian situation.