In a lightning-fast march that has astonished Bashar al-Assad's government, Syrian rebels took over most of Aleppo, including an airbase used by Russian forces.

Photographs taken outside the mediaeval citadel in the heart of Syria's second city show fighters pushing deep into the city after breaching government lines in the west on Friday night.

The Syrian army acknowledged on Saturday that rebels had taken over a significant portion of the city, but claimed that government soldiers had temporarily withdrew in order to prepare for a counteroffensive.

It asserted that the departure was a part of a reorganisation attempt before reinforcements arrived to begin the counterattack.

Dozens of soldiers had been killed or injured in fierce battles with insurgents in Aleppo and Idlib over the past few days, the military said.

It came as the Kurdish-led Syrian Democratic Forces (SDF), took control of Aleppo’s international airport, in the east of the city, after Syrian regime forces left.

Video also appeared to show SDF reinforcements travelling towards the city.

The group, which received significant US backing to fight Islamic State militants, is not aligned with Hayat Tahrir al-Sham (HTS), the jihadist group leading the thrust into Aleppo, and analysts said it was unclear what would happen when the groups met.

Meanwhile, Russian forces were reported to have withdrawn from their base in the Al-Suqaylabiyah area in the northern Hama countryside towards the Russian military base at Hmeimim Airport in the Latakia countryside, Syria.

HTS fighters seized swathes of the city on Friday against almost no resistance as Syrian army forces melted away, monitors said.

Rami Abdel Rahman, director of the UK-based Syrian Observatory for Human Rights, told AFP that HTS and its allies “took control of most of the city and government centres and prisons”.

On Saturday, HTS released a video saying it had captured Abu al-Duhur Air Base, about 28 miles south of Aleppo. The air base changed hands and was destroyed earlier in the civil war. It has been held by the Syrian government since 2018.

The Assad regime was on Saturday rapidly trying to reinforce its remaining areas of control, while Russian aircraft launched air strikes to try to beat the rebels back.

They were accused of killing at least 20 people in an air strike on Aleppo on Saturday afternoon.

The attack struck Al-Bassel roundabout hours after rebels had toppled a statue of Bashar al-Assad’s late brother, Bassel al-Assad.

The offensive marks the most significant challenge in years to Assad, reigniting the front lines of the Syrian civil war that have largely been frozen since 2020.

Rebel groups were ousted from Aleppo in 2016 with the help of Russian air power and Iranian forces, after a brutal four-year siege.

Aleppo has not been attacked by opposition forces since they were last ousted from eastern neighbourhoods in 2016, following a gruelling military campaign in which Syrian government forces were backed by Russia, Iran and its allied groups.

Inside Aleppo, schools and government offices were closed as most people stayed indoors, according to Sham FM radio, a pro-government station. Bakeries reportedly remained open.

Social media posts purported to place the rebels outside the city’s citadel, and cellphone videos showed them visiting residents in their homes, seeking to reassure them they will cause no harm.

State media said reinforcements, as well as Russian arms and equipment, were on their way to repel the “terrorists”.

The HTS group, which had its roots in the local branch of al-Qaeda before turning on the extremists, said it had launched the assault in response to weeks of regime attacks on the rebel-held enclave in north-western Syria.

Turkey, which has backed Syrian opposition groups, failed in its diplomatic efforts to prevent the government attacks, which were seen as a violation of a 2019 agreement sponsored by Russia, Turkey and Iran to freeze the line of the conflict.

Turkish security officials said the rebels had originally intended a “limited” offensive toward Aleppo, from where attacks targeting civilians originated.

However, the offensive expanded as Syrian government forces began retreating from their positions, the officials said.