UN observers have confirmed the discovery of 13 bodies near the eastern city of Deir al-Zour.
The men were found with their hands behind their back and some had gunshot wounds in their heads.
The head of the UN observers, Major General Robert Mood, said he was ‘deeply disturbed’ by the ‘appalling and inexcusable act’.
"All the bodies had their hands tied behind their backs and some appear to have been shot in the head from a short distance," he said in a statement released on Wednesday.
He called on all parties to the Syrian conflict to "exercise restraint and end the cycle of violence for the sake of Syria and the Syrian people".
