A London-based Syrian human rights group said on Saturday that Syrian security forces had shot dead dozens of prisoners during a riot at a military jail near Damascus.
The Syrian Observatory for Human Rights said military police fired at Islamist prisoners who rioted at Sidnaya jail earlier in the day.
"The number of the dead are in the tens," the group said in a statement. "Prisoners have gone to the roof, fearing for their lives. Military police elements are still firing live bullets..."
There was no immediate word from Syrian authorities on the report.
"The Syrian Observatory for Human Rights demands that President Bashar Assad intervenes immediately to stop this massacre," it said.
Syria holds hundreds of Islamist militants and other political prisoners. International human rights groups often complain of random arrests in Syrian jails.
Shooting by the guards was still going on, the group said.