US said the reports underscored the need to remove the regime of President Bashar al-Assad in Syria. (Agencies)
However, Syria said the report had no credibility as it was commissioned by Qatar, which funds rebel groups.
Coming a day ahead of the peace talks are due to begin in Switzerland, the report, by three former war crimes prosecutors, is based on the evidence of a defected military police photographer, who along with others reportedly smuggled about 55,000 digital images of some 11,000 dead detainees out of Syria.
A spokesperson for the Syrian Ministry of Information, Bassam Abu Abdullah, questioned the report's evidence. He said it was unclear where the information had come from or if the photographs were "from Syria or from outside Syria".
Meanwhile, delegations are arriving for the talks, known as Geneva II, which open in Montreux on Wednesday, and continue in Geneva two days later.
US said the reports underscored the need to remove the regime of President Bashar al-Assad in Syria.