Amnesty International on Tuesday (February 7th) accused the Syrian regime of hanging up to 13,000 people at a notorious prison over five years in a "policy of extermination", AFP reported.

The watchdog's report, titled "Human Slaughterhouse: Mass hanging and extermination at Saydnaya prison" near Damascus, details ritual mass hangings between 2011 and 2015.

At least once a week, up to 50 prisoners were taken out of their cells for arbitrary trials, beaten, then hanged "in the middle of the night and in total secrecy", the report said.

"Throughout this process, they remain blindfolded. They do not know when or how they will die until the noose was placed around their necks."

Most of the victims were civilians believed to be opposed to the regime.

"They kept them (hanging) there for 10 to 15 minutes," a former judge who witnessed the executions said.

"For the young ones, their weight would not kill them. The officers' assistants would pull them down and break their necks."

Amnesty said the mass executions amounted to war crimes and crimes against humanity, but were likely still taking place.

Thousands of prisoners are held at the military-run Saydnaya prison, 30 kilometres north of Damascus, one of Syria's largest detention centres.

Amnesty accused the regime of carrying out a "policy of extermination" there by repeatedly torturing detainees and withholding food, water and medical care.

The watchdog has previously said more than 17,700 people were estimated to have died in regime custody in Syria since March 2011. That figure did not include the up to 13,000 executed in Saydnaya.

Amnesty said it gave the names of 87 prison officials and guards responsible for the atrocities to unspecified "bodies capable of conducting credible investigations" into the killings.

A UN investigation last year accused the regime of carrying out a policy of "extermination" in its jails.