BEIRUT, Lebanon — Syria’s divided city of Aleppo plunged back into the kind of all-out war not seen in months on Thursday, witnesses and health workers said, as they reeled from government airstrikes that demolished a hospital in the insurgent-held side and from retaliatory mortar assaults by rebels on the government-held side.

At least 27 people, including three children and six staff members, were reported killed in the strike on the hospital, which turned it into a smoking pile of rubble on Wednesday night, and 20 were reported killed in airstrikes on Thursday. At least 14 people, mostly civilians, were killed in the mortar attacks on government-controlled areas, said officials at a hospital where casualties were streaming in throughout the day on Thursday.

The deadly destruction in Aleppo punctuated a drastic escalation in fighting over the past week that has shattered a partial truce in a war that has consumed Syria for more than five years.

The escalation also threatened to derail renewed attempts at peace talks in Geneva by the United Nations, and could disrupt or stop humanitarian aid to besieged parts of the country, affecting millions of people, relief officials said.