Amnesty International says it has uncovered evidence of mass hangings, torture and starvation at a government-run detention facility in Syria.
The human rights organisation has dubbed the Saydnaya Military Prison complex, 18 miles outside Damascus, a "human slaughterhouse" where the regime has carried out an "extermination" of political opponents amounting to "crimes against humanity".
Contrary to the impression given by the prison's name, the victims are "overwhelmingly" civilians rather than rebel fighters and include political dissidents, aid workers, students and journalists.
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Syrian President Bashar al-Assad's secretive regime makes verifying accusations of human rights violations a challenge for humanitarian groups, but Amnesty workers were able to interview dozens of former Saydnaya detainees and their relatives, as well as guards, who have since fled Syria.
In their report into the prison, Amnesty describes torture, starvation and mass hangings carried out in the middle of the night after "trials" typically lasting less than three minutes.
"One former military officer said he could hear 'gurgling' as people were hanged in an execution room below," Al Jazeera reports.
Based on witness testimony, the organisation estimates that between 5,000 and 13,000 people were extrajudicially executed at Saydnaya between September 2011 and December 2015, and "there is no reason to believe that executions have stopped".
Reports of repression and torture by the Syrian government date back to the 1980s, but since the country's descent into civil war in 2011, the regime's violations against detainees "have increased drastically in magnitude and severity", Amnesty says.
Statistics compiled by the Human Rights Data Analysis Group indicate that between March 2011 and December 2015, at least 17,723 people were killed in Syrian government custody.
Lynn Maalouf, deputy director for research at Amnesty's Beirut office, said the report exposes a "hidden, monstrous campaign, authorised at the highest levels of the Syrian government".
She added: "The cold-blooded killing of thousands of defenceless prisoners, along with the carefully crafted and systematic programmes of psychological and physical torture that are in place inside Saydnaya prison cannot be allowed to continue."
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