Syrian authorities closed Aleppo airport and cancelled all flights on Saturday, three military sources told Reuters, as rebels opposed to Syrian President Bashar al-Assad said they had reached the heart of the city.

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.

Robert Ford, who was the last U.S. ambassador to Syria, said the attack showed that Syrian government forces are "extremely weak." In some cases, he said, they appear to have "almost been routed."

Russia, a key ally of Assad, has promised Damascus extra military aid to thwart the rebels, two military sources said, adding that new hardware would start arriving in the next 72 hours.

The rebels began their incursion on Wednesday and by late Friday an operations room representing the offensive said they were sweeping through various neighbourhoods of Aleppo. Residents fled neighbourhoods on the city's edge because of missiles and gunfire, according to witnesses.

WATCH | Militants launch offensive in Syria: 

Militants launch offensive in northwest Syria

A UN official says at least 27 civilians have died in recent days after a militant group, reported to be the Islamist group Hayat Tahrir al-Sham, launched attacks on towns and villages in Aleppo province.

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. 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.

Dareen Khalifa, a senior adviser with the International Crisis Group and an expert on Syrian groups, said the insurgents have signalled for a while that they were ready for an offensive. But no one expected the swift advance of the forces toward Aleppo.

"It's not only that the Russians are distracted and bogged down in Ukraine, but also the Iranians are distracted and bogged down elsewhere. Hezbollah's distracted and bogged down elsewhere, and the regime is absolutely cornered," she said.

"But the surprise element comes in with how quickly the regime crumbled."

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.

Biggest attack in years

The attack is the biggest since March 2020, when Russia and Turkey agreed to a deal to de-escalate the conflict.

On Friday, 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 spokesperson Dmitry Peskov said on Friday that 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.