Pro-regime militiamen swept through farmlands in central Syria slaughtering dozens including women and children, activists said Thursday, sparking opposition calls for increased military raids by armed rebels.
The Syrian Observatory for Human Rights said at least 55 people were killed in Wednesday's assault on Al-Kubeir, a small Sunni farming enclave surrounded by Alawite villages in the central province of Hama.
Most of the victims were from a single family.
The reported massacre, which Damascus denied had occurred, was condemned as "brutal and sickening" by British Prime Minister David Cameron and as "unconscionable" by US Secretary of State Hillary Clinton, who insisted it was time for President Bashar al-Assad to go.
At the United Nations, Secretary General Ban Ki-moon said the new Syria massacre was "shocking and sickening," and said UN monitors were shot at while going to Al-Kubeir.
The head of the UN observer mission in Syria, Major General Robert Mood, said Syrian soldiers and local civilians prevented his monitors from reaching the area.
"We are receiving information from residents of the area that the safety of our observers is at risk if we enter (the) village of Al-Kubeir," Mood said.
Syrian state television denied the monitors were being turned away, adding that authorities were helping them go anywhere they wanted.
Russia, which with China this week came out strongly against intervention and regime change in Syria, called the killings "provocations" aimed at undermining UN-Arab League envoy Kofi Annan's six-point plan to end the violence.
"There are 49 confirmed and identified victims in Al-Kubeir, the majority of them from the Al-Yateem family," said Rami Abdel Rahman, head of the Britain-based Observatory.
"Among the dead are 18 women and children," he said, adding that six other people were also killed on Wednesday in a village near Al-Kubeir, in a farming area northwest of Hama city.
Earlier reports from opposition groups had put the death toll at between 87 and 100.
The Observatory was joined by the exiled opposition Syrian National Council (SNC) and the Muslim Brotherhood in blaming the killings on shabiha militiamen loyal to Assad's regime.
They and activists, citing survivors and witnesses, said the militiamen stormed into the small settlement on Wednesday afternoon after regime forces had bombarded it.
They then went on a killing spree, hacking, stabbing and shooting residents.
A resident from a nearby village told AFP the charred bodies of women and children still lay across Al-Kubeir on Thursday.
"Burned bodies of children and women and girls were on the ground," he told AFP by telephone from near Al-Kubeir.
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"I saw something you cannot imagine. It was a horrifying massacre... people were executed and burned. Bodies of young men were taken away," said Laith, who gave only his first name for fear of retribution.
"They had guns and knives... They went there from nearby villages like Asileh, which is Alawite," he said of the offshoot of Shiite Islam from which Assad and his family hails.
A video posted on YouTube showed bodies of several children, including babies, wrapped in blankets and white plastic body bags, purported to be victims of the massacre. Some were charred beyond recognition.
The video's authenticity could not immediately be verified.
Damascus denied responsibility, saying in a televised statement: "What a few media have reported on what happened in Al-Kubeir, in the Hama region, is completely false."
"A terrorist group committed a heinous crime in the Hama region which claimed nine victims. The reports by the media are contributing to spilling the blood of Syrians."
The opposition called for stepped up military assaults against regime forces.
"The Syrian National Council calls on the (rebel) Free Syrian Army to step up military assaults on regime forces to break the siege against the civilian population and protect civilians throughout the country," Mohammed Sermini, spokesman for the coalition, told AFP.
The Al-Kubeir incident comes after at least 108 people were killed in a May 25-26 massacre near the central town of Houla, most of them women and children who were summarily executed, the United Nations said.
Cameron said the international community must do more to isolate and condemn Damascus.
"If these reports are true, it is yet another brutal and sickening attack, and frankly the international community has got to condemn absolutely this regime and President Assad for what he is doing," he said.
Clinton squarely blamed Damascus for the massacre.
"The regime-sponsored violence that we witnessed in Hama yesterday (Wednesday) is simply unconscionable," Clinton told a news conference.
She said a solution to the crisis required a ceasefire, a transfer of power and the formation of a representative interim government, setting the stage for a renewed diplomatic stand-off with Moscow and Beijing.
In Beijing, leaders of the Shanghai Cooperation Organisation group led by Russia and China issued a statement opposing military intervention in the Middle East and urging a "peaceful resolution of the Syrian problem through political dialogue."
In Moscow, Russian foreign ministry spokesman Alexander Lukashevich called the massacre "the most brutal and vile provocations to undermine the plan of Kofi Annan."
Nationwide on Thursday, the Observatory reported at least 15 people killed, 11 of them civilians.
Russia and China have vetoed two Security Council resolutions against Assad's regime, but backed Annan's blueprint to end the conflict in which the Observatory says more than 13,500 people have died since March 2011.