There is clear evidence that Syria has systematically tortured and executed about 11,000 detainees since the start of the uprising, a report by three former war crimes prosecutors says.
The investigators examined thousands of still images of dead prisoners reportedly smuggled out by a defector.
One of the authors told the BBC there was evidence of government involvement. Damascus has denied claims of abuse.
The report comes a day before peace talks are due to begin in Switzerland.
The Guardian newspaper in the UK – which along with CNN first unveiled the report – says the release appears timed to coincide with the conference, opening in the resort town of Montreux, and continuing in Geneva two days later.
The talks, known as “Geneva II”, are being seen as the biggest diplomatic effort to end the three-year conflict which has left more than 100,000 dead and millions displaced.
In its annual report, released on Tuesday, Human Rights Watch accuses Russia and China of allowing abuses to take place by blocking action through the UN. It also accuses both government and pro-opposition forces of human rights abuses including torture and extrajudicial killings.
The Geneva II peace talks, it says, “shouldn’t become the latest excuse to avoid action to protect Syrian civilians”.
The report by the former war crimes prosecutors was commissioned by Qatar, which supports Syrian rebel and opposition groups. It is based on the evidence of a defected military police photographer, referred to only as Caesar, who along with others reportedly smuggled about 55,000 digital images of some 11,000 dead detainees out of Syria.
He told investigators his job had been to take photographs of corpses, both to allow a death certificate to be produced and to confirm that execution orders had been carried out.
“There could be as many as 50 bodies a day to photograph which require 15 to 30 minutes of work per corpse,” he is quoted as saying.
He did not claim to have witnessed killings or torture himself, which the investigators said gave weight to his testimony.The photographs cover the period from the start of the uprising in 2011 until August last year.
All but one of the bodies shown were male. Investigators say most of the bodies were emaciated; many had been beaten or strangled. Some had no eyes, and some showed signs of electrocution.
One of the authors of the report, Prof Sir Geoffrey Nice, told the BBC’s Newsday programme that the scale and consistency of the killings provided strong evidence of government involvement that could support a criminal prosecution.
Forensic pathologist Stuart Hamilton told Newsday that in the images that he saw, a large number of detainees were showing “evidence of significant starvation”.
He said that many looked as if they had been bound or restrained.
“There were a large number who had been beaten. And there were a significant minority who had clearly been strangled,” he said.
The Syrian government has not commented on the report, but has denied accusations of human rights abuses during the 34 months of the conflict.
The BBC’s Paul Wood, who has visited Syria several times during the conflict, says claims of systematic and bureaucratic torture are the unifying story among the rebels.
The Syrian government and the main exiled opposition alliance, the National Coalition, are due to send delegates to the Geneva II conference, which begins on Wednesday.
On Monday, the UN’s secretary general withdrew an invitation to Iran – a key ally of the Assad government – over its refusal to endorsethe Geneva Communique, the plan for a transitional governing body agreed at a UN-backed meeting in 2012.
The invitation to Iran had angered the US, while the National Coalition had threatened to pull out if the invitation was not rescinded. It has since confirmed it will attend.
It is unclear whether Iran will be able to join the talks two days later, when they move from Montreux to Geneva.
Iranian Foreign Minister Mohammad Javad Zarif said Tehran regretted that the invitation had been withdrawn “under pressure”.
Russian Foreign Minister Sergei Lavrov said not inviting Iran was “a mistake”, but added: “There is no catastrophe, we will push for a dialogue between the Syrian parties without any preconditions.”
The conference is the culmination of months of diplomacy. In May last year, Russian and the US agreed to try to bring both sides together.