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.

Warplanes targeted the city of Idlib in an attempt to fend off advances made by antigovernment forces that included a stunning assault on Aleppo.

Russian and Syrian fighter jets have struck the rebel-held city of Idlib on Dec. 1 amid attempts to push back an insurgency that has taken over most of Aleppo, Reuters reported, citing Syrian army sources.

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 surprise attack, led by the Islamist group Hayat Tahrir al-Sham, was the boldest rebel assault for years in the civil war that has been frozen.

The Syrian government has been fighting back against opposition forces with airstrikes after they took over large swathes of land in a lighting advance which has caused embarrassment to President Bashar Al-Assad.

NBC News’ Matt Bradley reports on the intensifying violence in the Syrian rebel offensive after Russian and Syrian jets struck a rebel held city in the northern part of the country. Syrian President Bashar Assad is vowed to crush insurgents waging a shock offensive against his regime.

At least 372 people have been killed since rebels launched a surprise offensive on Wednesday, according to observers.