An explosion at a mosque in Syria's Idlib province on Friday killed at least 25 members of the Al Qaeda-linked Nusra Front, the British-based Syrian Observatory for Human Rights monitoring group said.

The blast in the town of Ariha in northwestern Syria went off as members of the Nusra Front gathered at the mosque for iftar, the meal with which Muslims break their fast during the holy month of Ramadan.

Observatory, which tracks the war, said the explosion in Salem Mosque in Ariha, also killed a senior non-Syrian member of the Nusra Front. Opposition-based social media websites had conflicting casualty figures with some saying more than 40 people were dead.

Turkey's state-run Ananolu Agency reported 15 deaths, citing a statement from Istanbul-based Syrian Revolution General Commission.

No group announced responsibility for the blast, but supporters of Nusra Front blamed rival self-proclaimed Islamic State of Iraq and Sham (ISIS) militants who have fought the group on several frontlines in Syria.

A coalition of rebel groups called Jaish al Fateh, or Conquest Army, in which Nusra Front is a major part of its composition, have controlled most of Idlib province since the capture of the provincial capital in March.

The region borders Turkey and neighbors President Bashar al-Assad's heartland in Latakia province on the Mediterranean coast.