Syrian rebels advanced against regime forces on Monday as an offensive aimed at breaking the siege of eastern Aleppo went into its second day, activists and a monitoring group said.

Heavy fighting took place overnight and into the morning as rebels launched their long-heralded assault on government positions in the south-west of Syria's largest city, the Syrian Observatory for Human Rights said.

The rebel forces on the south-western outskirts are separated by only a few kilometres of regime territory from rebel-held eastern Aleppo, which has been cut off from the outside world since government forces severed its last supply route in early July.

The assault is being spearheaded by Islamist rebels including the Fatah al-Sham front, the new name of former Syrian al-Qaeda branch al-Nusra Front which last week ended its formal affiliation with the international jihadist network.

"All kinds of weapons are being [used] and the fighters of Aleppo are pushing forward with God's will, and we will win in order to liberate our people in Aleppo," Fatah al-Sham activist Abul-Fadl said.

Observatory director Rami Abdel-Rahman said he expected the fighting "is going to be a hard battle" for both sides, with government forces using warplanes and heavy artillery to strike back.

Abdel-Rahman said the rebel offensive appeared to be aimed at capturing a highway in the Ramousiyeh district which is the main government supply route for western Aleppo.

The rebel offensive was large-scale but highly unlikely to succeed, analyst Fabrice Balanche of the Washington Institute for Middle East Policy said.

By concentrating their forces for the offensive the rebels were presenting a target for airstrikes, while without any air support of their own they had little hope of success, he told dpa.

Aleppo-based activists posted pictures of smoke from burning tyres over the contested areas as rebels sought to limit visibility and hamper airstrikes.

Syrian state media confirmed battles were taking place between the Syrian army and rebels but denied that the rebels had made any advances.

An estimated 250,000 to 300,000 civilians still live in besieged eastern Aleppo. The UN has warned that food supplies there are expected to run out by the middle of the month.

Government forces were likely to impose a lengthy siege on eastern Aleppo rather than engage in risky urban combat, Balanche said, and if the current rebel offensive failed rebel forces inside the city would eventually "give up hope" and seek to negotiate an exit.

On Thursday, the Syrian government said it had opened three humanitarian corridors for residents to cross into the government-held western sector.

However, opposition activists said the routes were not safe, with the opposition Syrian Civil Defence warned that it had documented at least one case of a civilian being killed by regime forces while attempting to cross.

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