Residents in rebel-held areas of Syria's Aleppo cowered in their homes on Saturday as relentless missile strikes and barrel bomb attacks pounded the besieged eastern half of the divided city.

The raids by Russian and Syrian aircraft continued for a second night, after Damascus announced an operation late Thursday to recapture all of the city.

Heavy bombardment on Friday killed at least 47 people, among them seven children, the Britain-based Syrian Observatory for Human Rights monitor said.

The toll was expected to rise with the ongoing strikes levelling entire buildings, and obliterating whole streets.

‘There are certainly deaths in the bombing (on Saturday) but we don't have tolls yet and people are still trapped under the rubble,’ said Observatory director Rami Abdel Rahman.

An AFP correspondent in eastern Aleppo saw massive destruction in several neighbourhoods, including Al-Kalasseh and Bustan al-Qasr, where some streets were almost erased by the bombardment.

Unexploded rockets were still buried in the roads in some areas, and elsewhere enormous craters around five metres (16 feet) deep and wide had been left by the bombing.

Residents and activists described the use of a missile that produced earthquake-like tremors upon impact and razed buildings right down to the basement level where many residents desperately seek protection during bombing.

The civil defence organisation known as the White Helmets was left overwhelmed by the scale of the destruction, particularly after several of its bases were damaged in bombing on Friday.

The group says it has just two fire engines left for all of eastern Aleppo which, like its ambulances, are struggling to move around the city.

With no electricity or fuel for generators, the streets of Aleppo are pitch black and difficult to navigate at night, and the fuel shortage has also made it tough to fill up vehicles.

In many places, rubble strewn across streets has rendered them impassable and has effectively sealed off neighbourhoods to traffic.

On Saturday morning, the streets were nearly empty, with just a few residents out looking for bread.

The approximately 250,000 people in eastern Aleppo have been under near-continuous siege since government troops encircled the area in mid-July.

A truce deal negotiated between Moscow and Washington brought a few days of respite from the violence, though no humanitarian aid, earlier this month.

But the deal has fallen apart, and on Thursday the Syrian army announced an operation to retake all of Aleppo, urging civilians in the east to distance themselves from ‘terrorists’ and promising them safe passage to government-controlled areas.