Syrian authorities killed at least 13,000 people in mass hangings at a prison north of Damascus known to detainees as the "slaughterhouse" in the years following the 2011 uprising, Amnesty International said Tuesday.



The group released a report covering the period from the start of the uprising until 2015, during which Amnesty says groups of 20 to 50 people were hanged at Saydnaya Prison, once or twice a week in the middle of the night. It said the killings were authorized by senior officials, including deputies of President Bashar Assad.



"The horrors depicted in this report reveal a hidden, monstrous campaign, authorized at the highest levels of the Syrian government, aimed at crushing any form of dissent within the Syrian population," said Lynn Maalouf, deputy director for research at Amnesty's regional office in Beirut.



She told The Associated Press the detainees were only told they were sentenced to death "a few minutes before the noose is tied around their neck."



"These executions take place after a sham trial that lasts over a minute or two minutes, but they are authorized by the highest levels of authority," including the Grand Mufti, a top religious authority, and the defense minister, she said.



Amnesty said its findings are based on interviews with 31 former detainees and more than 50 officials, including prison guards and judges.



Amnesty said torture and abuse is hardly limited to Saydnaya, which is run by the military police. In a report released last year, the group documented over 17,000 deaths from torture and abuse in detention facilities across Syria.



Syrian security forces have long faced allegations of brutality, and their harsh crackdown on protests in 2011 led many opponents of the government to take up arms. The resulting civil war has killed an estimated 400,000 people.



Syrian officials rarely comment on widespread allegations of torture in the country's notorious prisons, but they have denied accusations of mass killings.



Amnesty called on the Syrian government to cease extrajudicial killings and torture in Saydnaya, one of the country's largest prisons. It also called on Russia and Iran, the Syrian government's closest allies, to press it to end its "calculated campaign" against dissent.