A new report from Amnesty International has said that up to 13,000 detainees at a prison near Damascus have been hung after confessions given under torture and trials that lasted just minutes. The executions were said to have taken place at Saydnaya Military Prison over four years.

Many more are thought to have died from the conditions in the prison, with bodies thought to be piled in mass graves on nearby military land.

"Saydnaya Military Prison is where the Syrian state quietly slaughters its own people," says the report, adding that the inmates are mostly those thought to oppose Syrian President Bashar Al-Assad's government. It adds that detainees were executed secretly in the dead of night "after being repeatedly tortured and systematically deprived of food, water, medicine and medical care".

Amnesty also points a finger at the Assad regime itself: "It is inconceivable that these large-scale and systematic practices have not been authorised at the highest levels of the Syrian government."

The organisation says that its investigation estimates between 5,000 to 13,000 people were killed in Saydnaya between September 2011 and December 2015, and that though they do not have evidence of executions after that, "There is no reason to believe the executions have stopped."

NPR reported that 84 people were interviewed for the investigation, among them 31 former Saydnaya detainees. Amnesty also said they had attempted to contact the Syrian government but had received no response.

There have long been allegations of human rights abuses, notably torture, by the Assad regime. However, The Times reports that this is the first evidence of extra-judiciary killings in prisons.

One former detainee told the investigation that they would hear beatings taking place at night: "At first we were thinking the people were being released or taken to the civilian prisons. But at midnight, we heard the sound of torture again, and we thought they were dying, because the sound of the torture was so strong. They were beating them in a monstrous way."