Russian and Syrian warplanes have intensified attacks on rebels who over-ran most of Aleppo, Syria’s second city, in a lightning assault that poses the biggest challenge in years to Bashar al-Assad’s regime.

Air raids struck the rebel-held city of Idlib for a second day on Sunday, while opposition media and war monitors said Russian and Syrian jets had also launched attacks near Aleppo University Hospital.

Thousands of rebels, led by Islamist group Hayat Tahrir al-Sham, entered Aleppo city, which has a population of 2mn, on Friday. Images circulating on opposition-linked social media this weekend showed them raising their flag over the city’s citadel and posing in its airport.

The rebels, who launched their assault only on Wednesday, said their fighters had advanced in multiple directions from their stronghold in Idlib province in north-western Syria, although their progress seemed to have slowed by Sunday.

HTS rebels attempted to press on to the major regime-held city of Hama, south of Aleppo, and claimed they had seized at least four towns in Hama province. The Syrian army has denied this. Rebels also said they captured the strategic town of Sheikh Najjar.

In his first public comments since the start of the offensive, Assad said Syria would continue to “defend its stability and its territorial integrity in the face of terrorists and their supporters”, in remarks carried by state news agency Sana.

The comments came during a call with Emirati leader Mohammed bin Zayed, an Assad ally, who “emphasised the UAE’s solidarity with Syria and its support in combating terrorism”.

Later, Assad vowed to defeat the insurgents in a phone call with the acting leader of the breakaway Georgian region of Abkhazia, Badra Ganba, saying “terrorism only understands the language of force”.

It was not clear whether Assad had returned to Syria following a visit to Moscow earlier this week.

The Syrian army denied that the rebels had secured Aleppo, but later said it was redeploying its forces as it prepared to launch a counterattack supported by Russian air strikes and strengthen its defensive lines after days of fierce fighting. Dozens of Syrian army soldiers were killed by rebel forces, the defence ministry said.

Social media videos showed rebel fighters driving through the streets of Aleppo, pulling down and kicking statues of Assad family members and celebrating by honking horns and firing their weapons. Videos also showed them freeing captives from Aleppo prisons.

Hundreds of civilians fled the city and its suburbs and headed for regime- or Kurdish-controlled areas, fearing a repeat of the gruelling 2016 battle that devastated their city. Those who remained in Aleppo were placed under night-time curfew by the HTS, residents told the Financial Times, adding that the streets were mostly empty on Sunday.

Assad faces increasing domestic and external pressures in a country shattered by a civil war that erupted after a 2011 popular uprising. He was able to quash the original rebellion with military backing from Russia, Iran and Iran-backed groups, including Hizbollah, the Lebanese militant movement.

Despite regaining control over two-thirds of the country, years of conflict and a deep economic crisis have left much of Syria in ruins.

The fighting had largely subsided in recent years, with the surviving rebel groups pushed into northern and north-western areas close to the Turkish border.

But over the past year, Israel has intensified air strikes on Iran-affiliated targets in Syria as it launched an offensive against Hizbollah in Lebanon, severely weakening groups that had played a vital role in keeping Assad in power.

HTS’s ability to move deeper into Syria is a major embarrassment for Assad, underscoring the regime’s weakness. The offensive appeared to have been planned for years, and comes at a time when Assad’s allies are preoccupied with their own conflicts in Ukraine and the Middle East.

HTS leader Abu Mohammad al-Jolani said his fighters would not rest “until we reach the heart of Damascus”, in old video footage that was republished by social media networks linked to the group this weekend.

Russian warplanes bombed rebel positions in a bid to stem their advance. Russia’s defence ministry was quoted by state news agencies as saying the country’s forces had killed “at least 300 militants by missile strikes . . . on command posts, warehouses and artillery positions”.

Russian military blog Rybar, known to be close to the defence ministry, said it understood that Major General Sergei Kisel, the top commander of Russia’s forces in Syria, had been removed from his post.

He was earlier removed from his position commanding Russia’s 1st tank army in Ukraine shortly after the start of Russia’s full-scale invasion in 2022, after the supposedly elite force suffered heavy defeats in the first weeks of the war.

The Rybar channel also said Russian troops were forced to evacuate the Kuweires air base in the Aleppo area as the rebels advanced. The Syrian air base was regularly used by Russian forces.

Russian foreign minister Sergei Lavrov has discussed the situation in Syria with Hakan Fidan, his Turkish counterpart. Lavrov also spoke to Iran’s foreign minister Abbas Araghchi, who is expected to visit Damascus on Sunday and Ankara on Monday, as the main powers involved in Syria began a flurry of diplomacy.

Araghchi on Sunday reaffirmed Iran’s unwavering support for Assad, accusing radical Islamist factions of aligning with the interests of the US and Israel. “We will witness their defeat,” he said.

Iranian state media reported that opposition forces in Aleppo seized Iran’s consulate on Saturday, tearing down and destroying images of Iranian political and military leaders.

The Kurdish-led Syrian Democratic Forces, which are supported by the US in the fight against Isis and control swaths of Syria’s north-east, announced a general mobilisation on Sunday. They called on people to join in the defence against the rebel offensive, which they say was “orchestrated” by Turkey — their longtime foe.  

Additional reporting by Najmeh Bozorgmehr in Tehran