"Islamist prisoners started a riot inside the prison this morning," the Syrian Observatory for Human Rights said in a statement on Saturday, citing a conversation with a political prisoner in Saydnaya jail.

"Shooting is continuing against the prisoners," the group said, adding that some prisoners fled to the roof of the military prison to escape the violence.

The group initially said 10 prisoners had died, but a spokesman later put the toll at 25.

The group said it had received phone calls from relatives of prisoners asking Bashar al-Assad, Syria's president, to help bring an end to the violence.

There was no immediate comment from the Syrian authorities.

Agencies