Syria's army advanced overnight deeper into east Aleppo where it now controls more than half of the former militant stronghold after a fierce assault that has sparked an international outcry.

Thousands of civilians have fled eastern neighbourhoods of the battered city since President Bashar al-Assad's government began its latest offensive in mid-November.

Overnight, government troops and allied forces seized the district of Tariq al-Bab where heavy fighting had raged a day earlier, the Syrian Observatory for Human Rights said Saturday.

The government has now recaptured around 60 per cent of eastern parts of the city that the militants overran in mid-2012, according to the Britain-based monitor.

The advance opens the road leading from the government-controlled west of the city to the international airport just outside Aleppo to the east, which is also held by the government forces.

Syrian armed forces have made swift gains in east Aleppo, and its loss would be the biggest blow yet to Syria's militants in the more than five-year-old war.

More than 300,000 people have been killed since the conflict started with anti-government protests in March 2011, and over half the country's population has been displaced.

Militants have struggled to hold back government ground forces, who have advanced backed by air strikes and artillery fire.

On Friday, they rolled back some regime gains in the Sheikh Saeed district on Aleppo's southeastern outskirts, but it was unclear how long they could hold that line.

Sheikh Saeed borders the last remaining parts of Aleppo still in militant hands.

In preparation for street-by-street fighting in these districts, hundreds of fighters from Syria's elite Republican Guard and Fourth Division arrived in Aleppo Friday, the Observatory said.

On Friday, as the army advanced in Tariq al-Bab, an AFP correspondent said residents had emptied out of neighbouring Shaar district, anticipating the arrival of fighting there.

'Race again time'

Moscow, a staunch ally of the Syrian government, has proposed setting up four humanitarian corridors into east Aleppo.

“We have informed the UN in New York and Geneva that there is no longer a problem with the delivery of humanitarian cargo to eastern Aleppo,” Russia's Foreign Minister Sergei Lavrov said Friday.

He said the UN was coming up with a plan and approval from Syrian authorities remained essential.

Moscow has announced several humanitarian pauses in Aleppo to allow civilians to flee, but until the recent escalation, only a handful did so.

“What is critical now is that we provide the immediate and sustained assistance that these families desperately need,” UNICEF spokesman Christophe Boulierac said.

“It's a race against time, as winter is here and conditions are basic. “