At least 25 inmates were shot dead by Syrian security forces during a riot in a jail for political prisoners in the mountains outside Damascus on Saturday, according to a human rights group.

"Islamist prisoners started a riot inside the prison this morning," the Syrian Observatory for Human Rights said in a statement received in Nicosia, quoting a political prisoner in the jail contacted by mobile phone.

"Shooting is continuing against the prisoners," the London-based group said, adding that a number of inmates had climbed the roof of the military prison in Saydnaya, north of Damascus, to escape the violence.

It said the number of dead was now 25.The group said it had received phone calls from relatives of prisoners asking Syrian President Bashar al-Assad to intervene to stop the clashes in Saydnaya, an ancient town with biblical connections.

There was no immediate comment from the Syrian authorities.