Syria's army advanced in east Aleppo Saturday in a devastating assault that has placed it in control of more than half the former rebel stronghold and sparked a civilian exodus.

Tens of thousands of civilians have fled eastern neighbourhoods of the battered city since President Bashar al-Assad's regime 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.

On Saturday, regime forces also secured a zone around the road from regime-held west Aleppo to the international airport just east of the city, taking two whole districts while fighting continued in a third.

The government has now recaptured around 60 percent of eastern parts of the city that rebel forces seized in mid-2012, the monitor said.

The advance has prompted more civilians to flee, heading either further south into remaining rebel-held districts or crossing into areas under regime or Kurdish control.

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

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

- 'Chasing terrorists' -
Around 310 civilians have been killed in the government's assault on east Aleppo since November 15, according to the Britain-based Observatory.

At least 69 civilians have been killed in the same period by rebel fire into west Aleppo, it says.

The government has trumpeted its advances, with state television on Saturday showing buses full of residents going from west Aleppo back to their homes in neighbourhoods retaken by the army.

"The army is advancing bit by bit according to plan," Brigadier General Samir Salman said.

He said it was determined to "carry out its job of chasing terrorists in the eastern districts and elsewhere", referring to the rebels.

Rebel forces have struggled to hold back government ground units, who have advanced backed by air strikes, barrel bombs and artillery fire.

On Friday, the insurgents 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 is close to the last parts of Aleppo still in rebel hands -- densely populated residential neighbourhoods where thousands have sought refuge from advancing regime forces.

The Observatory said rebels brought down a Syrian air force plane overnight near Aleppo airport, killing both pilots.

In preparation for street-by-street fighting, hundreds of elite Republican Guard and Fourth Division troops arrived in Aleppo on Friday, it said.

- 'Time to talk' -
The fighting has forced more than 50,000 people to leave eastern Aleppo for territory held by either the government or Kurdish forces.

Nearly 20,000 of them are children, according to UN estimates.

On Saturday, the Observatory said three people were killed in air strikes on Shaar, and raids were also targeting other eastern neighbourhoods.

Five people were killed by rebel fire on west Aleppo, the Observatory and state news agency SANA said.

The escalating violence has been met with international outrage, including a UN warning that east Aleppo could become "a giant graveyard".

On Saturday, top EU and UN diplomats warned there could be no victory in the battle for Aleppo without negotiations to ensure a viable future for Syria.

"You can win a war but you can lose the peace," EU foreign affairs head Federica Mogherini said.

UN Syria envoy Staffan de Mistura said he hoped a regime victory in Aleppo would not lead "the government to say: 'We won the war, and therefore no need for negotiations'".

"Now it's time for negotiation, but negotiating in real terms, which means power sharing... Otherwise, the alternative could be no major conflict but a creeping, ongoing guerilla (war) and no reconstruction," the envoy said.

Moscow, a key regime ally, has proposed setting up four humanitarian corridors into east Aleppo but said regime approval remained essential.

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

Many civilians in the east previously expressed fear of leaving to government-held areas or through passages run by Moscow, which began a bombing campaign in support of Assad's forces in 2015.

Both Damascus and Moscow have accused rebels of using civilians as "human shields".