Syrian officials could face war crimes charges based on photographs from a defector proving the "industrial-scale" torture and killing of 11,000 detainees by the regime, international prosecutors say.
Evidence smuggled out by a former Syrian military police photographer was reminiscent of the conditions in the death camps in Nazi Germany in the second world war, the three investigators said.
A report by the prosecutors - commissioned by Qatar, which backs the Syrian rebels - provides "clear evidence" of the starvation, strangulation and beating of detainees in President Bashar al-Assad's prisons.
The release of the report on Tuesday, first revealed by Britain's The Guardian newspaper and CNN, came a day before talks were due to begin in Geneva aimed at negotiating an end to Syria's bloody civil war.
"There is clear evidence, capable of being believed … in a court of law, of systematic torture and killing of detained persons by the agents of the Syrian government," the report said.
"Such evidence would support findings of crimes against humanity against the current Syrian regime. Such evidence could also support findings of war crimes against the current Syrian regime."
Syria has previously denied torturing detainees. A spokesman for UN rights chief Navi Pillay said further investigation was "clearly necessary".
The report was written by Desmond de Silva, former chief prosecutor of the special court for Sierra Leone; Geoffrey Nice, the former lead prosecutor in the trial of former Yugoslavian president Slobodan Milosevic; and David Crane, who indicted Liberian president Charles Taylor.
The defector, identified only as "Caesar" for his own safety, presented forensic experts commissioned by a London legal firm representing Qatar with about 55,000 digital images of 11,000 dead detainees since the start of the uprising in Syria in 2011.
The images were on memory sticks. He claims the victims all died in captivity before being taken to a military hospital.
De Silva said the report was the "smoking gun" showing evidence of "industrial-scale" killing by the Syrian regime.
"The pictures of emaciated bodies are reminiscent of the sort of pictures one saw after the second world war when the Nazi concentration camps were opened," he said.
The report says that all but one of the victims were male and most appeared to be aged between 20 and 40. The defector photographed as many as 50 bodies a day, the report said.