Warplanes targeted rebel-held areas of eastern Aleppo, Syria on Friday in a second day of heavy bombardment hours after the army announced the start of a military operation there, rescue workers and activists said.

The Syrian military, which is backed by the Russian air force, said late on Thursday it was starting a new operation against the rebel-held east, which is home to at least 250,000 people and was also targeted in heavy air strikes on Thursday.

The Syrian military could not immediately be reached for comment on Friday, and there was no word on casualties.

Ammar al Selmo, the head of the civil defence rescue service in eastern Aleppo, told Reuters a squadron of five warplanes was in the skies over the city, identifying them as Russian.

A fresh wave of bombing had started at from 6 a.m. (0300 GMT), after heavy overnight attacks, he said. "What's happening now is annihilation," he said.

The Syrian Observatory for Human Rights reported at least 30 air strikes had targeted different areas of Aleppo from midnight.

Overnight Thursday bombardment of rebel-held eastern districts of the city killed 45 people, director of al-Quds hospital Dr. Hamza al-Khatib told Reuters.

Warplanes mounted the heaviest airstrikes in months against rebel-held districts of Aleppo overnight, rebel officials and the Syrian Observatory for Human Rights said earlier.

U.S. Secretary of State John Kerry demanded on Wednesday that Russia and the Syrian government immediately halt flights over Syrian battle zones, in what he called a last chance to salvage a collapsing cease-fire and find a way "out of the carnage".