The Syrian military's bombardment damaged a water pumping station and rebels shut down another in retaliation, the UN 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.
"There are planes in the sky now," Ammar al Selmo, the head of Civil Defence in the opposition-held east, told Reuters from Aleppo on Saturday.
"Our teams are responding but are not enough to cover this amount of catastrophe."
Rebel officials said heavy airstrikes on Saturday by Russian jets hit at least four areas in the east, home to more than 250,000 people.
The British-based Syrian Observatory for Human Rights said the 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. Mr Selmo 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.
SkyNews Reports
SkyNews Reports