A new wave of airstrikes on rebel-held areas of Aleppo has killed at least 30 people, as nearly two million civilians go without running water in the devastated city.

A UN spokesperson said the Syrian military's bombardment damaged a water pumping station and then rebels shut down another in retaliation.

Residents said Aleppo has been subjected to the most ferocious bombardment of the war since the government declared a new offensive in the aftermath of a ceasefire brokered by the US and Russia breaking down.

Rebel officials said heavy airstrikes on Saturday by Russian jets hit at least four areas in the east.