At least 25 people were killed in northwestern Syria in air strikes carried out by the Syrian government and Russia, the Syrian opposition-run rescue service known as the White Helmets said early on Monday.

Russian and Syrian jets struck the rebel-held city of Idlib in northern Syria on Sunday, military sources said, as President Bashar al-Assad vowed to crush insurgents who had swept into the city of Aleppo.

The army also said it had recaptured several towns that rebels had overrun in recent days.

Insurgents led by jihadi group Hayat Tahrir al-Sham launched a two-pronged attack on Aleppo and the countryside around Idlib last Wednesday, before moving toward neighbouring Hama province.

On Sunday, government troops created a “strong defensive line” in northern Hama, as they attempted to stall the insurgents’ momentum, according to Britain-based opposition war monitor the Syrian Observatory for Human Rights.

The surge in fighting has raised the prospect of another violent, destabilising front reopening in the Middle East at a time when Israel is fighting Hamas in Gaza and Hezbollah in Lebanon, conflicts that have repeatedly threatened to ignite an even wider regional war.

It also risks drawing Russia and Turkey — each with its own interests to protect in Syria — into direct heavy fighting against each other.

The insurgents announced their offensive last Wednesday, just as a ceasefire started between Hezbollah and Israel that raised some hope tensions in the region might be calming.

The surprise offensive is an embarrassment for Assad, and it comes at a time when his allies — Iran and groups it backs and Russia — are preoccupied with their own conflicts.

According to a statement from Assad’s office, Iranian Foreign Minister Abbas Araghchi reassured the Syrian leader during a visit to Damascus that Tehran was ready to support the government in their counter-offensive.

Arab leaders, including Jordan’s King Abdullah II and United Arab Emirates President Sheikh Mohammed bin Zayed Al Nahyan, have also called Assad to express their solidarity.

The civil war, which has killed hundreds of thousands of people and displaced many millions, has ground on since 2011 with no formal end.

Most of the heavy fighting stopped years ago after Iran-backed militias and Russian air power helped Assad win control of all major cities. (Agencies)