A report released by human rights group Amnesty International has accused the Assad regime of executing thousands of prisoners in mass hangings and conducting systematic torture in a military prison near Damascus.

The executions took place over a four-year span, though the report indicated that torture and mass executions are likely ongoing. The organization has called on the United Nations to conduct their own investigation into the situation, claiming that their evidence is sufficient to accuse the Syrian government of war crimes.

“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 International’s regional office in Beirut.

According to Amnesty International’s investigation, between 2011 and 2015, an average of 20 to 50 people were killed by hanging each week in a single prison—the Saydnaya military prison north of Damascus. Their report claims that between 5,000 and 13,000 people were executed in the prison in the four years immediately following the popular uprising in Syria that promptly descended into civil war.

“The victims are overwhelmingly civilians who are thought to oppose the government,” the report said. “Many other detainees at the Saydnaya military prison have been killed after being repeatedly tortured and systematically deprived of food, water, medicine and medical care.”

The report goes on to charge that the prisoners of the Saydnaya military prison were given “sham” trials before being sentenced to jail time. The executions were then carried out in secret, with the victims being buried in mass graves and the families of those killed never being notified.

“Before they are hanged, victims undergo a perfunctory, one- or two-minute procedure at a so-called military field court. These proceedings are so summary and arbitrary that they cannot be considered to constitute a judicial process,” Amnesty International stated via a press release.

They cite Russia and Iran specifically in their report, calling on each of Syria’s allies to condemn these executions.