Islamist insurgents have entered Syria’s second city of Aleppo in a shock assault, eight years after forces loyal to Damascus seized control of the city.

Fighters from Hayat Tahrir al-Sham (HTS) began a major offensive earlier this week from their base in the Idlib countryside, a slim strip of land in Syria’s north-west. It took only three days for the fighting to reach Aleppo, with insurgents capturing territory around the city’s outskirts for the first time in four years as Syrian government forces pummelled rebel-held areas.

Turkey’s Anadolu state news agency reported on Friday afternoon that the insurgents had entered Aleppo, while unverified images and video circulating online showed armoured vehicles and armed uniformed militants on its streets. The Associated Press said residents reported hearing missiles striking its outskirts.

The fighting over the last three days had killed 27 civilians, including eight children, David Carden, the UN deputy regional humanitarian coordinator for the Syria crisis, told Reuters.

The rebels have rapidly recaptured dozens of towns and villages in the Aleppo countryside, seizing a military base, weaponry and tanks from Syrian government forces, while some Turkish-backed Syrian rebel groups based elsewhere in north-west Syria joined the fighting.

The UN said Syrian government forces based in Damascus carried out at least 125 airstrikes and shelled areas across Idlib and western Aleppo controlled by the rebels in response to the offensive, killing at least 12 civilians and wounding 46 others, and displacing 14,000 people.

Syria has been promised extra Russian military aid to help the army thwart the assault, two Syrian military sources told Reuters on Saturday. Damascus expects new Russian military hardware to start arriving at Russia’s Hmeimim airbase near Syria’s coastal city of Latakia in the next 72 hours, the sources added.

HTS said on Friday that it had captured four more towns including Mansoura, five miles from the centre of Aleppo. Syria’s state news agency said four civilians were killed inside student accommodation in the city when it was struck by projectiles from insurgent forces.

“The regime’s lines of defence have crumbled, I think they were taken aback. No one anticipated how fast the rebels would reach towards the edge of Aleppo,” said Dareen Khalifa, of the nonprofit International Crisis Group.

She added that it remained unclear whether the rebel forces would be able to hold the swath of captured territory, or how Russian forces backing the regime of Bashar al-Assad in Damascus may respond.

Turkey’s foreign ministry called for calm in the region around Idlib, demanding an end to the strikes on the area. “It is of utmost importance for Turkey that yet another and greater instability is avoided and civilians are not harmed,” it said.

A popular uprising against Assad’s rule in 2011 was violently quashed and descended into a bloody civil war that has gripped the country for more than a decade. Assad has maintained a fragile grip on power with backing from Russia and Iran. The battle for Aleppo in 2016, in which forces loyal to Damascus regained control of the city, marked a watershed moment for Assad’s control of the country.

A delicate balance of power in Syria has been increasingly tested over the past year, however, amid increasing regional fallout from Israel’s battle with the Iranian proxy group Hamas in Gaza.

Israel has dramatically escalated airstrikes against Iranian forces stationed on the ground in Syria, carrying out more than 116 strikes on Syrian territory, according to the UN, and killing more than 100 people, while recent fighting in Lebanon has forced 500,000 people to flee into neighbouring Syria.

The increasing Israeli strikes have put Iranian forces in Syria on the defensive, allowing rebels to exploit a moment where various proxy forces backing Assad are more engaged elsewhere.

Khalifa said Moscow remained focused primarily on the fighting in Ukraine. “The Russians are distracted in Ukraine. They are less invested politically if not military in Syria,” she said. “It’s difficult to tell what the result of this offensive is going to be. The rebels think the other side is vulnerable, and they have leverage.”

The Kremlin spokesperson, Dmitry Peskov, said on Friday that Moscow regarded the rebel attack as a violation of Syria’s sovereignty and wanted the authorities to act fast to regain control.

Turkey, which backs rebel groups along Syria’s northern border but has sought recently to normalise relations with Assad, is yet to publicly intervene in the latest round of fighting.

HTS said it would target Iranian forces fighting alongside Syrian government troops as part of the latest offensive. Iran’s Tasnim news agency said a commander from the Revolutionary Guards was killed in western Aleppo late this week.

The fighting and airstrikes appeared to paralyse much of the fragile network of services across rebel-held territory in Idlib, forcing the closure of health services and other infrastructure that sustain millions seeking shelter there.