Syrian activists said the victims were army defectors killed by Assad's forces, but it was not possible to verify their accounts.

Outrage at last Friday's mass killings in the Syrian town of Houla, documented by UN monitors, prompted a host of Western countries to step up pressure on Syria on Tuesday by expelling its senior diplomats and pressing Russia and China to allow tougher action by the UN Security Council.

Yesterday's observer report underlined how a peace plan drafted by international envoy Kofi Annan has failed to stem bloodshed or bring Syria's government and opposition to the negotiating table.

Major-General Robert Mood, the Norwegian head of the observer mission, said the corpses had been found with their hands tied behind their backs and signs that some had been shot in the head from close range.

"General Mood is deeply disturbed by this appalling and inexcusable act," a statement issued by the observer mission said. "He calls on all parties to exercise restraint and end the cycle of violence for the sake of Syria and the Syrian people."

The observers said the 13 dead men had been found on Tuesday evening in Assukar, about 50km east of the city of Deir al-Zor.

UN peacekeeping chief Herve Ladsous said in New York on Tuesday that the Syrian army and "shabbiha" militiamen supporting Assad were "probably" responsible for the Houla massacre.

Syria has denied any responsibility and blamed Islamist "terrorists".