Aleppo airport and a number of major roads have been closed by Syrian authorities after anti-regime rebels claimed to have reached the heart of the city, according to reports.
It comes after opposition fighters, led by the Islamist militant group Hayat Tahrir al Sham, carried out a surprise sweep through government-held towns earlier in the week.
Russia, one of President Bashar al Assad’s key allies, has promised Damascus extra military aid to thwart the rebels, two military sources told Reuters news agency.
The sources added that the Syrian army had been told to follow “safe withdrawal” orders from the main areas of the city that the rebels have entered.
“Relentless attacks” over the past three days in northwestern Syria have killed 27 civilians, including eight children, a UN official said.
And they have now returned to the Aleppo for the first time since 2016, when Assad and his allies Russia, Iran, and regional Shi’ite militias retook it.
The opposition fighters have said the campaign is in response to stepped-up strikes in recent weeks against civilians by the Russian and Syrian air forces on areas in rebel-held Idlib.
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Opposition sources in touch with Turkish intelligence said Turkey, which supports the rebels, had given a green light to the attacks.
However, Turkish foreign ministry spokesperson Oncu Keceli said on Friday that Turkey sought to avoid greater instability in the region and had warned recent attacks undermined de-escalation agreements.
Insurgents have posted videos online showing themselves using drones in their advance for the first time. It is not clear to what extent they were used on the battlefield.
Videos geolocated by Sky News show armoured vehicles on the fringes of Aleppo and rebels celebrating at its western entrance.
Another shows a group of more than 10 men running through the streets. At least one of them appears to be armed.
The Syrian government has not commented on the rebels breaching the city limits.