Fighters of Syria's Al-Qaeda affiliate and its allies overran the last major government-held city in the northwestern province of Idlib on Saturday, a monitoring group said.

The rebels launched their attack on Jisr al-Shughur on Thursday, seeking to capitalise on their capture of the provincial capital last month and open up a strategic assault route to the regime's stronghold on the Mediterranean coast.

"Al-Nusra Front and the Islamist brigades .. took control of almost all the city," Syrian Observatory for Human Rights chief Rami Abdel Rahman told AFP. "Government forces fled."