The Syrian government has secretly killed between 5,000 and 13,000 people in mass hangings at a military prison, according to a new report by Amnesty International.

The executions occurred at Saydnaya Military Prison between 2011 and 2015 under circumstances that constitute crimes against humanity, Amnesty said. Its findings were based on interviews with 84 people, including former detainees, prison guards, and lawyers, and there's no reason to believe the executions have stopped, the human rights group warned.

"They come from all sectors of Syrian society," the report stated. "Many are demonstrators, long-time political dissidents, human rights defenders, journalists, doctors, humanitarian aid workers and students."

According to the report, Syrian government officials systematically arrest civilians, as well as military personnel who oppose the regime. They are then put on trial — a proceeding that lasts three minutes or less — and condemned to death, the report stated. On execution days, prisoners are beaten for hours, blindfolded, and transferred in the middle of the night to a basement of the prison where they are hanged.

It's a process that takes place once or twice a week, and each time between 20 and 50 people are hanged to death, the report stated.