Syrian President Bashar al-Assad has said a victory for his army in the battered second city of Aleppo would be a "huge step" towards ending the country's five-year civil war.

In an interview with Syrian daily Al-Watan to be published on Thursday, an early copy of which was seen by AFP, Mr Assad said defeating beleaguered rebels in Aleppo would not put an end to Syria's conflict.

"It's true that Aleppo will be a win for us, but let's be realistic -- it won't mean the end of the war in Syria," Mr Assad said.

"But it will be a huge step towards this end," he said.

In a blistering three-week offensive, Syrian government forces have seized about 80% of east Aleppo, a stronghold for rebel groups since 2012.

Increasingly cornered in a pocket of territory in the city's southeast, opposition factions have called for an "immediate five-day humanitarian ceasefire".

When asked about the possibility of a truce in Aleppo, Mr Assad said, "it's practically non-existent, of course."

"The Americans in particular are insisting on demanding a truce, because their terrorist agents are now in a difficult situation," Mr Assad told Al-Watan.

He said a rebel loss in Aleppo "will mean the transformation of the course of the war across Syria."

Aleppo was once known as the beating heart of Syria's commercial and cultural industries.

But since violence broke out there four years ago, the city has been left divided between rebels in the east and government forces in the west.

Rebel fighters, who took control of east Aleppo in 2012, have suffered a string of defeats in recent days, losing about 80% of their former territory in the city, according to the British-based Syrian Observatory for Human Rights monitoring group.

Regime forces scored an important victory when the rebels retreated from the Old City, the historic heart of Aleppo.

They extended their advances later in the day, seizing the Bab al-Nayrab, Al-Maadi and Salhin neighbourhoods, according to state media.

Syria's conflict erupted in March 2011 with widespread demonstrations but has since turned into a brutal multi-front war drawing in world powers.