An explosion in a mosque in northern Syria killed 25 members of Al Qaeda's Syrian affiliate, including one of its leaders, as they attended Ramadan prayers, a monitoring group said on Friday.

Director of the Syrian Observatory for Human Rights Rami Abdel Rahman said the death toll could rise as dozens of civilians were also injured in the explosion during Iftar prayers in the city of Ariha.

"Twenty-five members of Al-Nusra Front, including a leader of the jihadist group, died in an explosion inside a mosque in the city of Ariha, in Idlib province," the Britain-based observatory said.

It was not immediately clear what caused the explosion.

Activist group the Syrian Revolution General Commission said "hundreds of civilians were assembled for an Iftar gathering with the Al-Nusra Front in the Salem mosque in the west of Ariha when the blast went off during evening prayers".

Civilians were also among those killed while worshipping during the Ramadan, the group said, without giving further details.

Most of Idlib province is now under rebel control, after an alliance of opposition groups including Al-Nusra Front evicted regime forces from their last strongholds.

More than 230,000 people have died in Syria since anti-government protests erupted in 2011, precipitating a civil war pitting pro-government forces, rebels and jihadist groups against each other.-Reuters