BEIRUT: Air strikes destroyed a hospital and killed dozens of people in rebel-held areas of Syria's Aleppo including children and doctors, and the United Nations called on Moscow and Washington to salvage a “barely-alive” cease-fire.

The city of Aleppo is at the epicentre of a military escalation that has undermined peace talks in Geneva to end the five-year-old war and UN envoy Staffan de Mistura appealed to the presidents of the United States and Russia to intervene.

Six days of air strikes and rebel shelling in Aleppo, which is split between government forces and rebels, have killed some 200 people in the city, two-thirds of them on the opposition side, the Syrian Observatory for Human Rights says.

“The catastrophic deterioration in Aleppo over the last 24-48 hours” has jeopardised the aid lifeline that delivers supplies to millions of Syrians, said Jan Egeland, chairman of the UN humanitarian task force. “I could not in any way express how high the stakes are for the next hours and days.”

The Geneva talks aim to end a war that has killed more than 250,000 people, created the world's worst refugee crisis, allowed for the rise of Islamic State and drawn in regional and major powers but the negotiations have all but failed and a truce to allow them to take place has collapsed.

Winding up the Geneva talks, de Mistura said he aimed to resume them in May, but gave no date.

“Wherever you are, you hear explosions of mortars, shelling and planes flying over,” Valter Gros, who heads the International Committee of the Red Cross Aleppo office, said.

“There is no neighbourhood of the city that hasn't been hit. People are living on the edge. Everyone here fears for their lives and nobody knows what is coming next,” he said.

A Syrian military source said government planes had not been in areas where air raids were reported. Syria's army denied reports that the Syrian air force targetted the hospital.

The Russian defence ministry, whose air strikes have swung the war in favour of President Bashar al-Assad, could not immediately be reached for comment. Russia has previously denied hitting civilian targets in Syria where it launched air raids late last year to bolster its ally.

The British-based Observatory said 31 people were killed as a result of air strikes on several areas of opposition-held Aleppo on Thursday. In addition, it said at least 27 people were killed in the air strike on the hospital that was struck late on Wednesday. Rescue workers put the toll higher.

In government-held areas, rebel mortar shelling killed at least 14 people, the Observatory and Syria's state news agency SANA reported.