A report released Tuesday by Amnesty International claims the Bashar al-Assad regime hanged 13,000 political prisoners in secret during the nation's civil war.
Between 2011 and 2015, dozens of prisoners were hanged as they were told they were being transferred from Saydnaya prison near Damascus.
The report also shows that Assad's regime tortures and starves prisoners, killing "massive numbers" of people. Many have not committed any crime aside from opposing Assad.
The horrors depicted in this report reveal a hidden, monstrous campaign.
Amnesty International called on Syria to stop the extrajudicial killings immediately and for its allies Russia and Iran to call for the country to stop.
WATCH | Amnesty International released this video alongside its report, retelling the stories of former Saydnaya prison guards.
If you put your ears to the floor, you could hear ... a kind of gurgling... We were sleeping on top of the sound of people choking to death.
Some former prisoners said they were raped or forced to rape other prisoners. Prisoners are not allowed to make any sounds, and looking at a guard can be punishable by death.
I'm just a headline. The bad president, the bad guy, who is killing the good guys. You know this narrative.
The Syrian Center for Policy Research claims the war has killed 400,000 people, plus 70,000 people indirectly affected by the violence.
Assad has denied past allegations of torture.