The head of the United Nations observer mission in Syria has renewed his call for all sides to exercise restraint for the sake of Syria and the Syrian people".
The bodies of 13 men were discovered on Tuesday night in al-Sukar, east of Deir al-Zour. They had been bound and shot at close range, the BBC reports.
In a statement, Major General Robert Mood said: "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 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."
As many as 15,000 people have been killed since a revolt against the rule of President Bashar al-Assad began in March last year.
The wave of international expulsions of Syrian diplomats following a massacre of 108 people in the town of Houla, near Homs, is continuing.
Forty-nine children and 34 women were among those killed in Houla last Friday and Saturday.
Turkey has told all Syrian diplomats to leave and Japan has expelled the Syrian ambassador in Tokyo.
UN / Arab League envoy Kofi Annan has told Mr Assad the country is at a tipping point.