UN observers said 13 bodies had been discovered bound and shot in eastern Syria, days after a massacre of 108 civilians, nearly half of them children, ignited a world outcry.

Syrian activists said the victims were army defectors killed by President Bashar Al Assad’s forces.

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

Rebels on Wednesday gave Assad a 48-hour deadline to abide by an international peace plan to end violence or face consequences, a rebel spokesman said. The US Treasury also imposed sanctions on Syria International Islamic Bank in an attempt to block Damascus from the global financial system.

Wednesday’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.

“Maj. Gen. 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 UN observers said the 13 dead men had been found on Tuesday evening in Assukar, about 50km east of the city of Deir Al Zor.

Video footage posted by activists shows the bodies face down on the ground, hands tied behind their backs, with dark pools of what could be blood around their heads and torsos. Gen. Mood did not apportion any blame for the killings.

UN peacekeeping chief Herve Ladsous said in New York on Tuesday that the Syrian army and ‘shabbiha’ militiamen supporting Assad were “probably” responsible for massacring 108 people in Houla with artillery, tanks, small arms and knives. Syria denied any responsibility and blamed “terrorists” — its term for rebel forces.

Fierce fighting erupted on Wednesday between regime troops and rebels near Damascus and in Homs province, a watchdog said, while reporting 39 more people killed across Syria.

The Syrian Observatory for Human Rights said five people were killed in the Damascus suburb of Douma, a hotbed of anti-regime sentiment, while a civilian was shot dead in Daraya, in the same province.

Also in Damascus province, government troops opened fire on mourners attending a mass funeral in Al Diabiya, killing four, said the Britain-based watchdog.

In central Homs province, four people died at Qusayr, scene of earlier explosions, while another five people, including a regular soldier, were killed in the city of Homs, it said.

Two people were killed in the region of Hama when fighting took place in the town of Kafarzita between troops and rebels. Later, in Hama city, residents held a general strike to mourn the deaths of the two men.

In Jabal Al Zawiya, a rebel commander was killed in northeastern Idlib.

A sniper shot dead a man at Aleppo in the north, while unidentified gunmen killed an 18-year-old in eastern Deir Al Zor. A rebel fighter was also killed in the same province, the Observatory added.

UN officials said the estimated number of internally displaced Syrians has more than doubled to 500,000 since an April 12 ceasefire and the flow of refugees abroad has gathered pace again.

The number of internally displaced people, a Syrian Arab Red Crescent estimate given to the UN, was about 200,000 before the peace deal which both sides have broken.