US Secretary of State Hillary Clinton has expressed outrage over a Syria government assault that reportedly left some 200 dead.
Mrs Clinton said accounts of the government attack on the village of Tremseh provided "indisputable evidence that the regime deliberately murdered innocent civilians".
"We call for an immediate ceasefire ... to allow the UN to enter Tremseh," Clinton said. "Those who committed these atrocities will be identified and held accountable."
Anti-regime activists in Syria said that government gunners rained shells on a poor farming village before armed thugs moved in, leaving scores dead in what rebels claim is one of the worst single days of bloodshed in the uprising against President Bashar Assad's regime.
Accounts of the killings varied, as did estimates of the number of dead, with some reports placing the toll as high as 200.
One amateur video showed a mass grave that was three bodies wide and about 10 bodies long. The video's narrator called it "the first group of martyrs from the Tremseh massacre".
Activist claims and videos could not be independently verified, but the killings appear to be the latest in a string of offensives by Syrian forces attempting to crush the uprising against Assad.
The Obama Administration blamed the Syrian regime for the violence and said the killings should boost international backing for efforts to push him from power.
The head of the UN monitoring mission in Syria also singled out government forces for blame, saying they attacked from the air and land in "continuous violence".
Yet much remains unclear about what happened in Tremseh, such as why Assad's troops moved against the village and whether all the dead were civilians. One group said dozens of the dead were rebel fighters.
Syria has grown increasingly chaotic as the protests that began in March 2011 have been overshadowed by scores of rebel groups waging an insurgency against the government.
Rebel groups operate independently, and a string of suicide bombings has raised fears of al-Qaida involvement.
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