A bomb has exploded inside a mosque where al-Qaeda’s branch in Syria was holding a fast-breaking meal, killing at least 14, activists said.

The blast in the Salem Mosque in the northern town of Ariha reportedly occurred shortly after sunset last night when scores of Nusra Front members had gathered to break their fast during the Muslim holy month of Ramadan.

The Britain-based Syrian Observatory for Human Rights said the explosion killed 31 Nusra Front members including five commanders.

Syria-based activist Ahmad al-Ahmad said 15 Nusra Front fighters were killed and more than 30 wounded.

The differences in casualty estimates could not be immediately explained since the Nusra Front cordoned off the area.

Meanwhile, Syrian troops backed by members of Lebanon’s Hizbollah group continued a major offensive under the cover of intense air strikes to retake a rebel-held mountain resort.

Taking the rebel-held town of Zabadani would tighten Hizbollah’s grip on Syrian territories bordering Lebanon and would strengthen the Syrian government’s control of the Beirut-Damascus highway.

Zabadani has been held by rebels since shortly after Syria's crisis began in March 2011. The conflict has killed more than 220,000 people and wounded at least a million, according to the United Nations.