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, many reportedly smuggled out by a defector.
A Syrian spokesman said the report had no credibility as it was commissioned by Qatar, which funds rebel groups.
The report comes a day before peace talks are due to begin in Switzerland.
The talks, known as "Geneva II", will open in Montreux, and continue in Geneva two days later.
It is seen as the biggest diplomatic effort yet to end the three-year conflict which has left more than 100,000 dead and millions displaced.
Meanwhile, 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.
Geneva II, it says, "shouldn't become the latest excuse to avoid action to protect Syrian civilians".
The report by the former war crimes prosecutors 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 said 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.
The photographs cover the period from the start of the uprising in March 2011 until August last year.
All but one of the bodies shown are male. Investigators say most 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 many looked as if they had been bound or restrained.
The report says the images are "clear evidence" of "systemic torture and killing of detained persons by agents of the Syrian government".
However, a spokesman for the Syrian ministry of information, Bassam Abu Abdullah, questioned the report's evidence, telling the BBC it was unclear where the information had come from or if the photographs were "from Syria or from outside Syria".
He said he was "astonished" at the figure of 11,000 victims, saying it had not been raised before this report.
He said: "I doubt this report. We should check these photos. Who are these people? Where are the names? From which prisons? Who is this person who has the authority to have these photos?"
Mr Abdullah said the international courts should direct their questions to Qatar.
He said: "If Qatar financed this report, there is no credibility because Qatar is one of the states who financed international terrorism and who sent killers to Syria. We have professional killers inside Syria from around the world. We are defending ourselves."
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.
US President Barack Obama telephoned Russian counterpart Vladimir Putin on Tuesday to "discuss the issues of the conference", the Kremlin said.
On Monday, the UN's secretary general withdrew an invitation to Iran - a key ally of Syrian President Bashar al-Assad - over its refusal to endorse the 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 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 when they move to Geneva.
Iranian Foreign Minister Mohammad Javad Zarif said Tehran regretted that the invitation had been withdrawn "under pressure", while Russian Foreign Minister Sergei Lavrov said not inviting Iran was "a mistake".
He added: "There is no catastrophe, we will push for a dialogue between the Syrian parties without any preconditions."