At least 20 people have been killed in a Syrian government air strike on a hospital in the city of Aleppo.

The attack is the latest in an intensification of government assaults on the city, with at least 100 civilians killed in air strikes, shelling and rocket fire since Friday.

"In the last 48 hours, we have had an average of one Syrian killed every 25 minutes, one Syrian wounded every 13 minutes," the United Nations special envoy to Syria, Staffan de Mistura, said.

The renewed fighting has all but destroyed a delicate ceasefire that started at the end of February, with UN-led peace talks now in doubt.

De Mistura has urged the US and Russia to intervene to save them.