Hundreds of residents of rebel-held eastern Aleppo fled shifting frontlines, sources said, after an advance by the Syrian army and allied forces that rebels fear could split their most important urban stronghold in two.

The army and its allies took control on Saturday of the large Hanano housing district, on the northeast frontline of the besieged eastern part of Aleppo.

On Sunday they said they had captured the neighbouring district, Jabal Badro.

Neither area was heavily populated but the advance, accompanied by Russian and Syrian air strikes, has raised fears among the insurgents that the northern part of east Aleppo could be cut off from the southern part.

That would weaken their control over the east and bring more residents closer to frontlines.

Capturing all of Aleppo would be a major victory for Syrian President Bashar al-Assad after five and a half years of fighting that has killed hundreds of thousands of people and displaced 11 million others.

The Syrian Observatory for Human Rights, which monitors the conflict from Britain, said about 400 people travelled into government-controlled Hanano from neighbouring rebel-held districts, from where some were transferred to government-held western Aleppo.

Some are also crossing into an Aleppo district held by the Kurdish YPG militia, which has largely avoided fighting the Syrian government and has been spared air attacks.

The Observatory said about 30 families entered this district, Sheikh Maqsoud.

Russian news agencies, citing the Defence Ministry, said more than 900 civilians, including 119 children, had left Jabal Badro in the last 24 hours.

"We left Hanano because of the bombardment from the Syrian army during their advance, and the chlorine gas," a resident said.

He was waiting with his wife, mother and three children at a minibus stop, hoping to travel on to government-held west Aleppo.

He said Hanano had contained about 200-300 families, but they had come and gone throughout the war depending on the intensity of strikes.

Aleppo, which was Syria's biggest city before the war began in 2011, is divided between the government-held west and rebel-held east.

UN officials say at least 250,000 people are under siege in the east.

There were fierce clashes in areas adjacent to Hanano, the Observatory, rebel sources and Syrian state media said.

Rebel sources say they are fighting back with difficulty in the face of sustained aerial bombardment.