By Dominic Evans
BEIRUT, May 30 (Reuters) - U.N. observers said on Wednesday 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 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 U.N. monitors, prompted a host of Western countries 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 U.N. Security Council.
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.
"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 U.N. observers said the 13 dead men had been found on Tuesday evening in Assukar, about 50 km (31 miles) 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. Mood did not apportion any blame for the killings.
U.N. 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 Islamist "terrorists" - its term for rebel forces.
Trying to save his seven-week-old peace plan from collapse, U.N.-Arab League envoy Annan told Assad in Damascus on Tuesday that Syria was at a tipping point, but there was no let-up in violence, with more than 100 people killed the same day, according to the Syrian Observatory for Human Rights.
Diplomats said the U.N. Human Rights Council would meet in Geneva on Friday to consider the Houla killings, the fourth such grilling Syria has faced since an anti-Assad revolt erupted in March 2011, inspired by Arab uprisings elsewhere.
"It's all materialising very quickly. It's going to have huge support," said one official. The United States, Qatar, Turkey and the European Union led the push for the session.
Assad has so far proved impervious to international scolding and Western sanctions for his ferocious crackdown on peaceful demonstrators and armed insurgents, and has failed to return troops and tanks to barracks as required by the Annan plan.
However, the U.N. observers sent in to monitor a notional ceasefire were able to verify the horrors in Houla, producing a wave of world revulsion hard for Moscow and Beijing to ignore.
But China and Russia have stuck to their rejection of any intervention or U.N.-backed penalties to force Assad to change course, while backing Annan's peace drive, the only broadly accepted initiative to halt the bloodletting in Syria.
The West is averse to military intervention, although French President Francois Hollande said on Tuesday that could change if the U.N. Security Council backed it - something that is not possible unless veto-wielding members Russia and China allow it.
Turkey joined other countries including the United States, Britain, France and Germany in expelling Syrian diplomats in protest at the Houla massacre, saying unspecified international "measures" would follow if crimes against humanity continued.
But Iran, itself at odds with the West over its disputed nuclear work, decried foreign criticism of its Arab ally.
Iranian Parliament Speaker Ali Larijani accused U.S. and other Western officials of seeking to "prepare the ground for a new crisis" over Syria, Iran's Press TV website reported.
"The Majlis (parliament) condemns the opportunistic intervention of certain countries in Syria and the U.S. terrorist measures and unwise warmongering messages," he said.
Stung by the expulsion of its diplomats around the world, Syria told the Dutch chargee d'affaires to leave. She was one of the few senior Western diplomats left in Damascus.
Russia's Foreign Ministry said the expulsion of Syrian diplomats was "counter-productive".
Despite the diplomatic deadlock, Annan, a former U.N. secretary-general, is pressing on with his mission.
"It is important to find a solution that will lead to a democratic transition in Syria and find a way of ending the killings as soon as possible," he said after talks in Jordan. on Wednesday. "With goodwill and hard work, we can succeed."
Yet it is hard to see where a breakthrough might come from.
"China opposes military intervention and does not support forced regime change," Foreign Ministry spokesman Liu Weimin said, apparently responding to Hollande's suggestion.
Russia also reasserted its hostility to military action or to any further Security Council measures beyond a non-binding statement that it backed on Sunday condemning the Houla killings.
"We believe consideration in the Security Council of any new measures to influence the situation now would be premature," said Deputy Foreign Minister Gennady Gatilov.
Annan's deputy, Jean-Marie Guehenno, was to brief the council later in the day on his boss's talks with Assad.
Russia and China have twice vetoed Western-backed council resolutions condemning Syria's violent response to dissent.
"The West should not expect China and Russia's cooperation if it insists on dictating its own values and standards to the world ... It will instead find China and Russia standing in its way," said the Global Times, which is published in Beijing by the Communist Party's People's Daily.
Wary Western governments have so far avoided any direct or indirect military involvement in Syria, whose sectarian-tinged conflict has already sent refugees spilling into Lebanon, Jordan and Turkey and could further destabilise the Middle East. (Additional reporting by Tom Miles in Geneva, Sui-Lee Wee and Sabrina Mao in Beijing, Alissa de Carbonnel in Moscow, Marcus George in Dubai, Louis Charbonneau and Michelle Nichols in New York and Andrew Quinn in Washington; Writing by Alistair Lyon; Editing by Kevin Liffey)