Syrian rebels now control around half of Aleppo after their surprise offensive has left regime forces reeling.
Opposition fighters have advanced in and around Syria’s second-largest city, and now also claim to be in control of all of Idlib province.
Russian and Syrian jets have responded by bombing the rebels as they contend with the most serious challenge to President Bashar al Assad in many years.
Syria’s military says it has conducted a “temporary troop withdrawal” to prepare for a counteroffensive against “terrorists”.
Aleppo airport and a number of major roads have been closed, while thousands of cars were seen fleeing the city along a route that remains under government control.
The rebels, led by the Islamist militant group Hayat Tahrir al Sham, are said to be in control of around half of Aleppo – Syria’s second-largest city – according to the UK-based Syrian Observatory for Human Rights (SOHR).
Dozens of Syrian soldiers have been killed in fierce battles in Aleppo and Idlib in the past few days, the military has said.
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.
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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.
The Russian and Turkish foreign ministers have discussed the situation on the phone and expressed concerns over escalation, according to Moscow.
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.
Syria has experienced more than 13 years of civil war. Here are some of the key events since the conflict began.
2011: Anti-regime sentiment spreads across Syria after protesters are shot dead by government forces. Conflict breaks out between rebels and pro-Assad troops.
2012: Government forces relentless bombard rebel territory, particularly the besieged city of Homs.
2013: The Islamic State (IS) is founded and begins to grow in strength and territory. A chemical weapon attack in Ghouta, Damascus, killed hundreds. The UN found that sarin gas was used, but did not assign blame to the Assad regime or the rebels.
2014: IS declares a “caliphate” covering much of Syria and continues to fight both the Assad regime and the rebels. Foreign countries begin bombing IS strongholds.
2015: IS fighters seized the historic city of Palmyra, destroying monuments across the city, a UNESCO Heritage Site.
Russia, launches its first airstrikes in Syria. Moscow said they were targeting IS, but the West accused them of also attacking rebels to support Damascus. This proved to be a turning point in the conflict.
2016: Syrian troops, backed by Iran and Russia, recapture Aleppo – a significant blow to the rebels.
2017: IS is driven from Raqqa, the capital of the “caliphate” by the US-backed Syrian Democratic Forces – widely seen as a symbol of the terror group’s decline.
2018: At least 70 people died and hundreds were injured in an alleged chemical attack on Douma, eastern Ghouta. The US State Department called it “one of the worst chemical attacks in Syrian history”. The US, UK and France launched airstrikes against Assad targets in response.
2019: Turkey launches an offensive into north-eastern Syria.
2021: Daraa in southern Syria sees intense fighting after the region boycotted the presidential election. A ceasefire is agreed in September.
2022: IS militants attack a prison in north-east Syria during which 500 people are killed and some fighters escape.
2023: A devastating earthquake strikes Turkey and Syria, killing tens of thousands of people.
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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.
International correspondent
It had appeared to be a conflict frozen in time, but the operation launched with blistering speed by Syrian opposition fighters has turned all of that upside down.
It is also a potent reminder of the complexity of the Syrian conflict.
What was a stalemate has revealed that the regime of President Bashar al Assad may very well be standing on clay feet.
Whether the rebel push continues its momentum and transforms into a wider offensive remains to be seen – but its significance politically and militarily is immense.
