The Syrian Government has been accused of systematically killing thousands of people over five years at a prison north of Damascus.

Key points: Amnesty International report says between 5,000 and 13,000 prisoners killed over five years

Amnesty International report says between 5,000 and 13,000 prisoners killed over five years Those executed were subjected to sham trials lasting minutes before they were hanged

Those executed were subjected to sham trials lasting minutes before they were hanged Amnesty has labelled killings a crime against humanity

Amnesty International said between 5,000 and 13,000 people were hanged at the Saydnaya prison between 2011 and 2015, and it was likely the killings were still going on.

The organisation said it obtained evidence from 84 witnesses, including survivors, guards and other former officials from the prison during a year-long investigation.

"Every week, once or twice a week, there are between 20 and 50 prisoners taken out of their cells, told that they will be transferred to another prison, and then instead of that they are taken to a small room, huddled together and hanged to death," Amnesty deputy research director Lynn Maalouf said.

Most of the victims were civilians believed to oppose the Government, Amnesty said.

The group said victims were subjected to sham military trials lasting just a few minutes before being executed.

Photos from 2010 and 2016 show the growing number of graves at a cemetery near the military facility. ( Supplied: Amnesty International )

Witnesses recall 'rancid, bloody smell' inside prison

Before and after shots of Saydnaya detainee Mounir al-Fakir. ( Supplied: Amnesty International )

A former judge from the military court told Amnesty that when the hangings did not work, there were officers on standby to finish the executions.

"They kept them there for 10 to 15 minutes," the judge said.

"Some didn't die because they are light. For the young ones, their weight wouldn't kill them.

"The officers' assistants would pull them down and break their necks. Two officers' assistants were in charge of this."

Amnesty also released direct testimony from several survivors including Salam Othman, a lawyer from Aleppo, who detailed systematic torture.

"When we were beaten in front of the cells, I saw a lot of blood on the floor. There was a rancid and bloody smell," he said.

Anas Hamido on the day before his arrest and soon after his release. ( Supplied: Amnesty International )

Former detainee Anas Hamido told Amnesty: "When they bring the food — it sounds like a battle, in every cell.

"They take out the head of the cell, or two others, to be beaten. You hear sounds — your heart drops to the floor. You begin to shake, you can't control it."

Amnesty has branded the killings a crime against humanity and called on Syria's ally, Russia, to use its power on the Security Council to allow an independent investigation.

"These death sentences were approved by the Grand Mufti of Syria and by either the Minister of Defence or the Chief of Staff of the Army who are deputised to act on behalf of the President, Bashar al-Assad," Ms Maalouf said.