As many as 13,000 people were hanged by the Syrian government in a campaign of extra-judicial executions, according to a chilling new report by Amnesty International.

The figure relates to deaths at Saydnaya Prison between 2011 and 2015. According to Amnesty's report Human slaughterhouse: Mass hangings and extermination at Saydnaya prison, every week and often twice a week groups of up to 50 people were taken out of their cells and hanged.

The report also highlights "inhuman" conditions at the prison, including systematic deprivation of food, water, medicine and medical care. It says the practices are authorised at the highest levels of the Syrian government.

"The horrors depicted in this report reveal a hidden, monstrous campaign, authorised 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.

"We demand that the Syrian authorities immediately cease extrajudicial executions and torture and inhuman treatment at Saydnaya Prison and in all other government prisons across Syria. Russia and Iran, the government's closest allies, must press for an end to these murderous detention policies.

"The upcoming Syria peace talks in Geneva cannot ignore these findings. Ending these atrocities in Syrian government prisons must be put on the agenda. The UN must immediately carry out an independent investigation into the crimes being committed at Saydnaya and demand access for independent monitors to all places of detention."

Amnesty says there are strong reasons to believe the executions are continuing.

A previous report found that more than 17,000 people had died in prisons across Syria since the crisis began in 2011; the 13,000 deaths at Saydnaya are in addition to this.

Prisoners are condemned to death by a Military Field Court after hearings lasting one or two minutes.

A former judge from a Syrian military court told Amnesty International the "court" operates outside the rules of the Syrian legal system. "The judge will ask the name of the detainee and whether he committed the crime. Whether the answer is yes or no, he will be convicted... This court has no relation with the rule of law. This is not a court," he said.

Convictions are based on confessions extracted from detainees under torture.

Those who are condemned to death do not find out about their sentences until minutes before they are hanged.

A former judge said: "They kept them [hanging] there for 10 to 15 minutes. 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."

Detainees held in the building in the floors above the "execution room" reported that they sometimes heard the sounds of these hangings.

"If you put your ears on the floor, you could hear the sound of a kind of gurgling. This would last around 10 minutes... We were sleeping on top of the sound of people choking to death. This was normal for me then," said "Hamid", a former military officer arrested in 2011.

Many prisoners said they were raped or in some cases were forced to rape other prisoners.

Amnesty called for the international community to take action. "The cold blooded killing of thousands of defenceless prisoners, along with the carefully crafted and systematic programmes of psychological and physical torture that are in place inside Saydnaya Prison cannot be allowed to continue. Those responsible for these heinous crimes must be brought to justice," said Lynn Maalouf.