The killing of more than 90 people including 32 children in the Syrian town of Houla has sparked international outrage.
Accusing the Syrian government of violating international law, a joint statement by the UN Secretary-General, Ban Ki-moon and the UN-Arab League envoy, Kofi Annan — cited by the BBC — condemned the "appalling and brutal crime."
In a statement, French Foreign Minister Laurent Fabius said the international community must stop the “martyrdom of the Syrian people,” according to Bloomberg Businessweek. British Foreign Secretary William Hague said the UK would call for an “urgent” meeting of the UN Security Council.
US Secretary of State, Hillary Clinton, meantime, pledged that Washington would work with other countries to end what she called President Bashar al-Assad's "rule by murder and fear.”
However, the Syrian government has insisted it was not responsible for the massacre.
"We completely deny responsibility for this terrorist massacre against our people," foreign ministry spokesman Jihad al-Makdissi told a news conference on Sunday, Agence France-Presse reported.
He blamed "terrorists" for the massacre on Friday and Saturday and said the government had opened investigation.
"We have set up a military and legal committee to investigate," he said. "The results will emerge within days."
A team of UN observers who visited Houla Saturday said 92 bodies, 32 of them children younger than 10, had been counted after reports of an artillery bombardment by Assad's forces.
Videos posted online and purporting to be from Houla showed many dead and badly mutilated infants, the Guardian reported, adding that residents said some victims were killed with knives and others shot.
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According to the Guardian, former UN secretary general, Kofi Annan — who negotiated a peace plan in March that appeared to now be in tatters — was due back in Damascus on Monday to discuss the ongoing crisis with key officials.
The rebel Free Syrian Army said it could no longer commit to the Annan-brokered cease-fire unless the UN Security Council could ensure that civilians were protected.
In a statement, the rebels said Annan's plan was "going to hell" if urgent steps were not taken.
UK Foreign Secretary William Hague, meantime, has said he would lobby Russia, the regime's only major foreign ally, during a visit to Moscow starting on Sunday, the BBC reported.
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