UN monitors confirmed on Wednesday that the bodies of 13 people found in Syria were executed.
The men were found face down with their hands tied behind their backs in the eastern city of Deir al-Zour.
Some of them had been shot in the head. Major General Robert Mood, the head of the UN mission in Syria, called it an “appalling and inexcusable act”.
The discovery follows Friday’s massacre in Houla that left more than 100 dead; atrocities which Syria blames on terrorists.
Bashar Jafari, the Syrian ambassador to the UN, said Damascus had already opened an investigation.
Those who perpetrated this heinous crimes will be held accountable by Syrian law”, he told reporters in New York.
The Security Council met in closed session on Wednesday to discuss the conflict.
China and Russia say they will block any new initiative against Syria on the grounds that they oppose foreign intervention.