In a major breakthrough in the push to retake the whole city, regime forces on Saturday captured Masaken Hanano, which had been the biggest rebel-held district in eastern Aleppo.

On Sunday, the 13th day of the operation, they also took control of the adjacent neighbourhoods of Jabal Badra and Baadeeen and captured three others, the Syrian Observatory for Human Rights said.

Inzarat, Al-Sakan al-Shaabi and Ain al-Tall have all returned to regime hands and government forces have made large forays into Sakhur and nearby Haidariya, the monitor said.

It said government forces are "in control of most of the northern part" of Aleppo.

"The rebels have lost at least 30 percent of the territory they once controlled in Aleppo," Observatory chief Rami Abdel Rahman said.

The regime gains came as its aircraft pounded rebel positions and amid heavy clashes between the opposition and forces loyal to President Bashar al-Assad in the strategic Sakhur district.

Masaken Hanano was the first district the rebels took in the summer of 2012 in a move that divided the city into a rebel-held east and a regime-controlled west.

Around 250,000 civilians besieged for months in the east have faced serious food and fuel shortages.

The Observatory said that nearly 10,000 civilians had fled east Aleppo overnight Saturday -- at least 6,000 to the Kurdish-controlled northern district of Sheikh Maksoud, with the rest fleeing to government-held areas.

"It is the first exodus of this kind from east Aleppo since 2012," Abdel Rahman said.

- Regime sights on Sakhur -

Syrian state television broadcast images of a crowd of civilians including women and children gathered around green buses that it said had come to pick them up in Masaken Hanano.

One woman was shown pushing a stroller and many others carried plastic bags on their heads as bombardment was heard in the distance.

Official media said they were taken "by the army to safe areas".

Yasser al-Youssef, from the rebel group Nureddin al-Zinki, said opposition fighters were consolidating their positions in Sakhur.

"We are strengthening our positions to defend the city and residents, but the aircraft are destroying everything methodically, area by area," he said of a regime campaign of air strikes.

Sakhur lies on a stretch of just 1.5 kilometres (less than a mile) between west Aleppo and Masaken Hanano, now both regime-controlled.

If the regime takes control of Sakhur, east Aleppo would be split in two from north to south, dealing a further blow to the armed opposition.

The latest regime push comes after days of intense bombardment on the east, which has been pounded with air strikes, shelling and barrel bombs.

On Saturday, dozens of families fled Sakhur and Haidariya as regime raids and artillery killed at least 18 civilians in several districts, the Britain-based Observatory said.

At least 225 civilians, including 27 children, have been killed since the government’s latest assault on east Aleppo began on November 15.

- IS chemical attack -

Rebel forces also intensified rocket attacks on western districts overnight, killing at least four civilians and wounding dozens, the Observatory said.

Such attacks have killed a total of 27 civilians since the offensive began, among them 11 children.

The United Nations has a plan to deliver aid to Aleppo and evacuate the sick and wounded, which rebel factions have approved but which Damascus has not yet agreed. Guarantees are also needed from regime ally Russia.

Once a commercial and industrial hub, Aleppo has seen some of the worst fighting in Syria’s nearly six-year war.

The conflict broke out in 2011 with the brutal repression of anti-government protests, and has since evolved into a complex war involving different factions and foreign powers.

On Sunday, the Turkish army said that 22 pro-Ankara Syrian rebels were hit by a chemical gas attack from Islamic State group jihadists in northern Syria.

The Turkish army is backing the Syrian fighters in an unprecedented cross-border operation it says is targeting both IS and the Kurdish People’s Protection Units (YPG) militia, which Ankara considers to be a "terrorist" group.

The YPG is a key component of a US-backed Arab-Kurdish alliance fighting to oust IS from its de facto Syria capital of Raqa, after the jihadist group overran large parts of Syria and Iraq in 2014.

Syria’s war has killed more than 300,000 people and displaced more than half the population.—AFP