UN observers said today 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, 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 yesterday by expelling its senior diplomats, and to press Russia and China to allow tougher action by the UN security council.
Today's UN observer report underlined how a peace plan drafted by UN-Arab League envoy Kofi Annan has failed to stem bloodshed or bring Syria's government and opposition to the negotiating table.
Mr Annan's deputy Jean-Marie Guehenno told the security council that Syria's protesters "have lost fear and are unlikely to stop their movement", according to a diplomat with knowledge of the closed session.
Mr Guehenno said direct engagement between government and opposition was "impossible at the moment" and expressed "serious doubts over the commitment of Syrian authorities to the Annan plan", the diplomat said.
Major-General Robert Mood, the Norwegian head of the observer mission, said the corpses found in Assukar, about 50km east of the city of Deir al-Zor, had 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."
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.
Mr Mood did not apportion any blame for the killings.
UN peacekeeping chief Herve Ladsous said in New York yesterday 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, Mr Annan told Dr Assad in Damascus yesterday 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.
Syria's state news agency said pumping had been halted to an oil pipeline in eastern Syria after a bomb attack today.
Diplomats said the UN 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.
Dr Assad has so far proved impervious to international scolding and Western sanctions for his 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 UN 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 UN-backed penalties to force Dr Assad to change course, while backing Mr Annan's peace drive, the only broadly accepted initiative to halt the bloodletting in Syria.
Asked if Western and Arab countries were pressing Moscow to change its position, Russian president Vladimir Putin's spokesman Dmitry Peskov told Reuters: "Russia is a country with a consistent foreign policy and any pressure is hardly appropriate."
The West is itself averse to military intervention, although French president Francois Hollande said yesterday that this could change if the UN 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.
In an attempt to limit the Assad government's access to the global financial system, the US Treasury imposed sanctions on Syria International Islamic Bank for helping another blacklisted Syrian bank evade Western sanctions.
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, Dr Annan, a former UN 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 today.
"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 Mr 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.
Russia and China have twice vetoed Western-backed council resolutions condemning Syria's violent response to dissent.
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.
Reuters