A report compiled by three international war crimes prosecutors claims that the Syrian government is behind the "systematic killing" of approximately 11,000 detainees between March 2011 and August 2013.
The Guardian, which obtained access to the report, says that the source of the report's claims is a military police photographer who secretly worked with a Syrian rebel group before defecting and fleeing the country. In the process, the defector smuggled the images of "killed detainees" out of the country on memory sticks.
The defector, identified in the report as "Caesar," does not claim to have witnessed executions or torture himself. However, he describes the bodies of detainees, mostly young men, as being emaciated, blood-stained, and, in some cases, bearing signs of strangulation or electrocution.
The report claims that Caesar photographed as many as 50 bodies a day. The purpose of the photographs, according to the defector, was to allow a death certificate to be produced without allowing families of the deceased to see the body, as well as to confirm that orders to execute prisoners had been carried out. Families were usually told that their loved ones died from either a heart attack or "breathing problems."
The 31-page report was commissioned by a London-based law firm operating on behalf of the government of Qatar, which has financed and armed rebel groups in Syria and repeatedly called for Syrian President Bashar al-Assad to stand trial for war crimes. Two of the report's authors, Sir Desmond de Silva and David Crane, argued war crimes cases related to the 1991-2002 civil war in Sierra Leone, while the third author, Sir Geoffrey Nice, led the prosecution of former Serbian leader Slobodan Milosevic at The Hague.
The publication of the report is believed to be timed to coincide with this week's peace conference in Geneva, which has been convened with the hope of creating a transitional government.
British Foreign Secretary William Hague called the report "further evidence of the systematic violence and brutality being visited upon the people of Syria by the Assad regime. We will continue to press for action on all human rights violations in Syria, and for accountability for those who perpetrate them."
A leading human rights group accused the U.S. and other world powers Tuesday of undermining efforts to bring Assad before the International Criminal Court.
The New York-based Human Rights Watch said the U.S. had focused too strongly on bringing the warring parties together for peace talks at the expense of putting "real pressure" on Assad's regime to end atrocities and hold those responsible to account.
The group also accused Russia and China of shielding their ally Syria from concrete action at the United Nations, such as arms embargoes.
"We cannot afford to wait for the distant prospect of a peace accord before the killing of 5,000 civilians a month comes to an end," Kenneth Roth, executive director of Human Rights Watch, told reporters as he presented the group's annual report in Berlin. "The mass atrocities being committed in Syria should be a parallel focus of the peace process."
Human Rights Watch said the United States had "its own reasons" for opposing the referral of Syria to the Hague-based International Criminal Court, including concerns that this could have legal implications for U.S. ally Israel. But Washington has also been reluctant to lean on Russia for fear of jeopardizing diplomatic efforts to bring all parties together for a peace conference in Switzerland this week.
Roth said he hoped the report would push U.S. Secretary of State John Kerry to call for an immediate end to atrocities in the same way that a chemical arms attack near Damascus last year resulted in swift international action.
"The pictures will make it that much harder to ignore the mass atrocities being committed by Assad," Roth told The Associated Press. "So far Kerry's only answer to the atrocities has been: `Oh we're trying to build peace; when we have peace the atrocities are going to stop.' But that's not an adequate answer when peace is going to be a long time coming and people are being killed every single day."
Human Rights Watch identified a new trend it called "abusive majoritarianism" last year. The idea implies authoritarian rule by a government that has, or claims to have, an overwhelming democratic mandate which it uses to suppress minorities.
The group cited the Muslim Brotherhood and subsequent military government in Egypt as a prime example, but also included the suppression of dissent in Turkey and anti-gay measures in Russia and Africa.
Roth also slammed as insufficient President Barack Obama's announcement to reform how the National Security Agency conducts its surveillance.
"[Obama] didn't move past this discredited, out-of-date view that we have no privacy rights in the metadata because we shared it with the phone company or the Internet company," Roth said.
The Associated Press contributed to this report.