A new wave of airstrikes on rebel-held areas of Aleppo has killed at least 25 people, as nearly two million civilians go without running water in the devastated city.

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

Hanaa Singer, a UNICEF representative in Syria, said intense attacks damaged the Bab al-Nairab station, which supplies water to the 250,000 people in rebel-held eastern part of Aleppo.

Ms Singer said the Suleiman al-Halabi pumping station, also located in the rebel-held east, was then switched off in retaliation - cutting water to 1.5 million people in government-held western parts of the city.

"Depriving children of water puts them at risk of catastrophic outbreaks of water-borne diseases," Ms Singer said.

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.

The British-based Syrian Observatory for Human Rights said Saturday's death toll of 25 is expected to rise because people remain trapped under rubble.

Residents spoke of many buildings being completely destroyed.

"They are using weapons that appear to be specifically for (bringing down) buildings," a senior official in an Aleppo-based rebel faction, the Levant Front, told Reuters.

"Most of the victims are under the rubble because more than half the civil defence has been forced out of service."

Seven of those killed on Saturday died in a strike as they queued to buy yoghurt at a market.

The Syrian Observatory for Human Rights said 47 people were killed on Friday, including seven children.

Ammar al Selmo, the head of Civil Defence in the opposition-held east, said the toll was more than 100.

Dramatic footage emerged of a five-year-old girl being pulled alive from the rubble of a Aleppo building destroyed in an airstrike on Friday.

The rest of her family - her mother, father, three sisters and one brother - were all killed in the airstrike.

The ceasefire negotiated by the US and Russia brought a brief respite from the violence, but the agreement broke down this week.

A Syrian military source told Reuters the operation announced late on Thursday was going to plan, as the army prepares for a ground offensive in a bid to recapture the rebel-held areas of the city.

When asked what weapons were being used, the source said the army was using precise weapons "suitable for the nature of the targets being struck, according to the type of fortifications", such as tunnels and bunkers, and "specifically command centres".

The Syrian army says it is targeting rebel positions, and denies hitting civilians.

"Every missile makes an earthquake we feel regardless of how far off the bombardment is," one resident said.