According to the Syrian Observatory for Human Rights, the fighting was among the deadliest in years, with 255 people killed. Most of the dead were combatants, but 24 civilians were also killed, most of them killed in Russian airstrikes.

The offensive began on Wednesday, the same day a fragile ceasefire between Israel and the Iran-backed militant group Hezbollah came into effect in neighboring Lebanon.

By Friday, the jihadists and their allies had seized control of more than 50 towns and villages in the north, marking the government’s biggest loss of territory in years, according to the Britain-based Observatory.

They then entered western districts of Aleppo, a city of around two million people that was Syria’s manufacturing center before the war.

“Hayat Tahrir al-Sham (HTS) and allied factions… were able to enter the outskirts of Al-Hamdaniya and New Aleppo neighborhoods… after carrying out two suicide attacks using two booby-trapped cars,” the war monitor said.

HTS, a jihadist alliance led by al-Qaeda’s former affiliate in Syria, shelled a student dormitory in the city, killing four civilians, state media reported.

According to the Observatory, Syrian and Russian warplanes launched heavy airstrikes on the rebel enclave around Idlib, where the jihadists are based, carrying out 23 airstrikes.

Army reinforcements have arrived in Aleppo, a Syrian security official told AFP, requesting anonymity to discuss sensitive matters.

An army statement said troops repelled the attack on the city and recaptured some positions.

The United Nations Office for the Coordination of Humanitarian Affairs said “more than 14,000 people – almost half of them children – have been displaced by the violence.”

Aleppo resident Sarmad, 51, told AFP he could hear “the sounds of rockets and artillery fire around the clock.”

“We are afraid that war will break out and we will be driven from our homes again,” he said.

Nasser Hamdo, 36, who works in a pastry shop, said he had been following the news since hostilities began.

On Thursday, the jihadists and their allies cut off the highway between Aleppo and the capital Damascus, some 300 kilometers south, the Observatory said.

“We fear that blocked roads could lead to an increase in fuel prices and prevent goods from reaching the city,” Hamdo said.

Kremlin spokesman Dmitry Peskov described the situation in Aleppo on Friday as “a violation of Syria’s sovereignty.”

He expressed support for “the Syrian government to quickly restore order in this district.”

Iranian Foreign Minister Abbas Araghchi pledged “continued support for the government, the nation and the army of Syria” in a phone call with his Syrian counterpart Bassam al-Sabbagh, according to a statement.

There has been a ceasefire brokered by Turkey and Russia in the Idlib region since 2020. The ceasefire was repeatedly broken but largely respected.

An AFP correspondent in the rebel enclave saw jihadists advancing in tanks as heavy gunfire erupted in an area just seven kilometers (just over four miles) from Aleppo.

AFP images showed abandoned army tanks and other military vehicles.

The correspondent said the jihadists and their Turkish-backed allies received orders from a joint operations command.

Analyst Nick Heras of the New Lines Institute for Strategy and Policy said the militants were trying to “preempt the possibility of a Syrian military campaign in the Aleppo region.”

According to Heras, the Syrian government and its main backer Russia had been preparing for such a campaign.

Russia intervened in Syria in 2015, turning the tide of the civil war that had broken out four years earlier in favor of the government, whose forces had by then lost control of most of the country.

Other interests are also at stake.

In addition to Russia, Syrian President Bashar al-Assad has also been supported by Iran and allied militant groups, including Lebanon’s Hezbollah.

Iran-backed militias have a large presence in the Aleppo region, having provided crucial ground support to the army as it recaptured rebel-held areas of the city in 2016.

Heras said anti-government forces were “better able to capture and occupy villages than Russian-backed Syrian government forces, while the Iranians are focused on Lebanon.”

The Observatory’s director, Rami Abdel Rahman, said government forces were “completely unprepared” for the attack.

“It is strange to see such heavy blows being dealt to regime forces despite Russian air support and early signs that HTS would launch this operation,” Abdel Rahman said.

“Were they dependent on Hezbollah, which is now active in Lebanon?”