Activists say Syrian opposition fighters have detonated a large amount of explosives in a tunnel under an army post in the northwestern province of Idlib, inflicting heavy casualties on the government troops.

Idlib-based activist Ibrahim Ismail says the blast was followed by intense clashes as the rebels launched an attack to retake the Arbaeen Mountain area early on Friday.

The Britain-based Syrian Observatory for Human Rights said the fighting is some of the most intense in months, adding dozens of Syrian troops have been killed and wounded.

The Observatory said the main rebel factions taking part in the attack include Suqour al-Sham as well as the al-Qaida-linked Nusra Front.

Last month, the Nusra Front and other rebel groups captured two major army bases in the province.