"The workers were on a bus when they were forced to stop at a checkpoint on the outskirts of Qusayr" late on Thursday afternoon, said Salim Kabbani of the Local Coordination Committees, which organise protests on the ground.
"Regime forces tied their hands behind their backs and shot them."
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UN rights chief Navi Pillay warned Syria and the entire region are in danger if a full-fledged conflict erupts in the country as she urged a probe into the Houla massacre.
In a statement to a specially-convened meeting of the Human Rights Council, Pillay also called on the international community to throw its weight behind the six-point peace plan brokered by UN-Arab League envoy Kofi Annan.
"Otherwise, the situation in Syria might descend into a full-fledged conflict and the future of the country, as well as the region as a whole could be in grave danger," said the High Commissioner for Human Rights.
Pillay, who is not in Geneva, reiterated a call for the UN Security Council to consider referring events in Syria to the International Criminal Court.
"We must make all efforts to end impunity, to ensure accountability for perpetrators, and to provide adequate and effective remedies for the victims," she said.
Pillay also voiced regret at the failure of Damascus to grant access to the Commission of Inquiry on Syria, the independent panel established last year by the rights council to investigate rights abuses.
The council was due on Friday to debate a call for the commission to probe the massacre of over 100 people in the Syrian town of Houla last week that triggered global horror and outrage.
Qatar, Turkey and the US have submitted a draft resolution which condemns the "outrageous" killing of 49 children in Houla and calls for a "comprehensive, independent and unfettered special inquiry".
President Vladimir Putin is to hold talks with German Chancellor Angela Merkel and French President Francois Hollande on his first foreign tour since returning to the Kremlin for a third term amid unprecedented protests against his rule.
Germany, France, Britain, the United States and other Western nations expelled Syrian diplomats in protest at the slaughter of 108 people, including women and children, many of whom were summarily executed according to the UN.
Syria allies China and Russia, which have both blocked previous attempts at the UN Security Council to condemn Damascus, joined other council members on Sunday in backing a statement condemning the Houla killings.
Moscow has insisted that Sunday's rebuke went far enough.
But US Secretary of State Hillary Clinton on Thursday warned that Russia's policy of propping up the Assad regime could contribute to a civil war and even lead to a wider proxy war because of Iran's support for Damascus.
The United States also condemned as "reprehensible" Russian arms deliveries to Syria, where activists say 13,000 people have been killed since Assad's regime launched a brutal crackdown on the opposition in March last year.
But Moscow has indicated it is in no mood to shift its stance on its Soviet-era ally, with Putin's spokesman Dmitry Peskov saying that Russia's position was "balanced and consistent and completely logical."
Hollande has refused to rule out foreign military intervention, as long as it is carried out with UN backing, while Germany -- which is not a permanent member of the Security Council -- wants all diplomatic avenues pursued.
On the eve of her talks with Putin, Merkel struck a conciliatory note, saying Russia had worked "constructively" on the Syria crisis at the Security Council.
German Foreign Minister Guido Westerwelle said in an interview with Die Welt newspaper Friday that Russia should recognise that efforts to halt the violence in Syria were "not working against Russian strategic interests".
"We must not give the impression in this difficult situation that military intervention is the road to a quick fix," he said.
Hollande, who will meet Putin for the first time since both men assumed office last month, said he would seek to persuade Putin to ramp up pressure on Syria and support a new round of sanctions.
"An armed intervention is not excluded on the condition that it is carried out with respect to international law, meaning after deliberation by the United Nations Security Council," Hollande said on Tuesday.
US Defence Secretary Leon Panetta stressed on Thursday that any military action in Syria would need UN backing.
The Kremlin is unlikely to radically shift its stance on Syria although Foreign Minister Sergei Lavrov has insisted that Russia supports the peace plan of UN-Arab League peace envoy Kofi Annan, and not Assad.
Intense behind-the-scenes negotiations focused on ways Moscow could back down without losing face are believed to be under way.
The New York Times has reported that one option promoted in some Washington and Moscow circles involves a transition of power similar to the one used to end Yemeni president Ali Abdullah Saleh's rule this year.
The plan would reportedly see Assad cede power to his inner circle for an interim period during which political talks with the opposition would be held.
Analysts said Moscow could theoretically back this option because its historic relations with Syria depend not on Assad but on generations of military and trade contacts that it might potentially keep in the long term.
Putin kicked off his three-nation tour on Thursday in isolated Belarus which is blackballed by the West for its government's crackdown on the opposition.
