Syria's army and allied forces seized a new neighbourhood from rebels in east Aleppo overnight, a monitor said Monday, and now hold around two-thirds of the former rebel bastion.

The army seized the Qadi Askar neighbourhood after earlier capturing the Karm al-Myessar, Karm al-Qatarji and Karm al-Tahan neighbourhoods, the Syrian Observatory for Human Rights said.

The Britain-based group said the advances left the large Shaar neighbourhood effectively encircled by government forces and in danger of falling from rebel control.

With the capture of Shaar, the army would hold 70 percent of east Aleppo, four years after rebels first seized it.

Observatory director Rami Abdel Rahman said hundreds of people had fled from the newly recaptured neighbourhoods.

The group estimates at least 50,000 people have left eastern neighbourhoods for areas under government or Kurdish control in the west or north of the city in recent days.

Others have fled south to the remaining territory held by the rebels in the city.

Syria's army began a new offensive to recapture Aleppo on November 15, and has pounded the east relentlessly with air strikes, barrel bombs and artillery fire since then.

An AFP correspondent in the east described a sleepless night for terrified residents with dozens of rockets hitting rebel-held neighbourhoods and the air filling with the smell of gunpowder.

He said residents had switched off all their lights, even turning off generators used in the absence of electrical supply, because they were attracting bombardment.

He added that residents were going down into basements, where available, or cowering in the entryways of buildings in the hope of remaining safe during the heavy bombardment.

The Observatory says at least 319 people have been killed in the government offensive in east Aleppo, including 44 children.

Rebel fire on the government-held west of the city has killed 69 people, including 28 children, in the same period, the monitor says.

East Aleppo has been besieged by government forces since mid-July, with international aid stocks exhausted and food supplies dwindling.

Most medical facilities in the east have been hit in government strikes, and rescue worker centres have also been struck.