Warplanes mounted a new wave of heavy air strikes on rebel-held areas of Aleppo on Saturday, rebel sources, a rescue worker and a war monitor reported, pressing a major offensive by the Russian-backed Syrian military to take back the whole city.

Residents of rebel-held eastern Aleppo say it has been subjected to the most ferocious bombardment of the war since the Damascus government declared a new offensive that has killed dozens of people in the last two days.

"Unfortunately it continues. There are planes in the sky now," Ammar al Selmo, the head of Civil Defense in the opposition-held east, told Reuters.

Rebel officials said heavy air strikes on Saturday morning hit at least four areas of the opposition-held east, home to more than 250,000 people. The attack has drawn on ordinance more destructive than anything previously used against the area, and many buildings have been entirely destroyed, residents say.

"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 defense has been forced out of service."

The Syrian Observatory for Human Rights, a British-based organization that reports on the war, said it had documented 47 deaths since Friday, including five children. Selmo said the toll was more than 100.

"The raids are intense and continuous," Observatory Director Rami Abdulrahman told Reuters.

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