The opposition fighters, led by the Islamist militant group Hayat Tahrir al-Sham, carried out a surprise sweep through government-held towns this week and reached Aleppo nearly a decade after having been forced out of the northern Syrian city.
Russia, a key ally of Assad, has promised Damascus extra military aid to thwart the rebels, two military sources said, adding new hardware would start arriving in the next 72 hours.
The rebels began their incursion on Wednesday and by late Friday local time an operations room representing the offensive said they were sweeping through various neighbourhoods of Aleppo.
They are returning to the city for the first time since 2016, when Assad and his allies Russia, Iran, and regional Shi'ite militias retook it, with the insurgents agreeing to withdraw after months of bombardment and siege.
Mustafa Abdul Jaber, a commander in the Jaish al-Izza rebel brigade, said their speedy advance this week had been helped by a lack of Iran-backed manpower in the broader Aleppo province.Â
Iran's allies in the region have suffered a series of blows at the hands of Israel as the Gaza war has expanded through the Middle East.
The rebels say the lack of Iranian-backed defences has helped their advance. (AP PHOTO)
Opposition sources in touch with Turkish intelligence said Turkey had given a green light to the offensive.
But Turkish foreign ministry spokesperson Oncu Keceli said Turkey sought to avoid greater instability in the region and had warned recent attacks undermined de-escalation agreements.
The attack is the biggest since March 2020, when Russia and Turkey agreed to a deal to de-escalate the conflict.
Syrian state television denied rebels had reached the city and said Russia was providing Syria's military with air support.
The Syrian military said it was fighting back against the attack and had inflicted heavy losses on the insurgents in the countryside of Aleppo and Idlib.
David Carden, UN deputy regional humanitarian co-ordinator for the Syria crisis, said: "We're deeply alarmed by the situation unfolding in northwest Syria."
"Relentless attacks over the past three days have claimed the lives of at least 27 civilians, including children as young as eight years old."
Syrian state news agency SANA said four civilians including two students were killed on Friday in Aleppo by insurgent shelling of university student dormitories. It was not clear if they were among the 27 dead reported by the UN official.
Kremlin spokesman Dmitry Peskov said Moscow regarded the rebel attack as a violation of Syria's sovereignty.
"We are in favour of the Syrian authorities bringing order to the area and restoring constitutional order as soon as possible," he said.
