Syrian army attack on Tremseh condemned by world leaders
US Secretary of State Hillary Clinton has suggested the Syrian army "deliberately murdered civilians" in the village of Tremseh.
Up to 200 people are reported to have died in an army attack on the village, which would make it the bloodiest single event in the Syrian conflict.
However, reports suggest that a large number of the casualties were rebel fighters.
The Syrian army admitted killing a "big number of terrorists" but no civilians.
UN Secretary General Ban Ki-moon said the attack casts "serious doubt" on President Assad's commitment to a peace plan.
UN observers confirmed that they saw signs of intense fighting in the area on Thursday, including the use of tanks, helicopters and heavy artillery.
The team is trying to arrange a ceasefire so it can go to the village to make a full analysis.
The UN Security Council is currently locked in debate about the future of the UN observer mission in Syria, which is set to come to an end on 20 July.
Western nations want a ten-day ultimatum to end violence with the threat of sanctions if the demands are not complied with to be part of the Security Council resolution on the future of the mission.
China and Russia remained opposed any moves to threaten Damascus with further sanctions.
The BBC's Jim Muir says the attacks at Tremseh have clearly had a political impact, but will probably not be enough to change Russia's opinion on sanctions.
Following the Tremseh attacks, Ban Ki-moon called on UN member states "to take collective and decisive action to immediately and fully stop the tragedy unfolding in Syria".
"Inaction becomes a license for further massacres," he said.
Hillary Clinton said there was "indisputable evidence that the regime deliberately murdered innocent civilians".
She insisted that "those who committed these atrocities will be identified and held accountable".
The US, France and UK all condemned the violence and called for co-ordinated action from the UN Security Council.
It is unclear what sparked the violence in Tremseh.
Early accounts from activists said government forces surrounded the village on Thursday in a bid to retake it from the rebel Free Syrian Army.
Troops bombarded Tremseh for several hours before pro-government militias from nearby Alawite villages moved in, killing many more villagers and setting fire to houses, activists said.
Others who tried to flee through fields were also gunned down, they claimed.
But later, activists told the AFP news agency that rebel fighters had attacked an army convoy, but were beaten back and many were killed in a counter-attack.
"At this stage, though we do not yet have the final count, the number of civilians killed by shelling is not more than seven," said Jaafar, an activist from the anti-regime Sham News Network.
"The rest were members of the Free Syrian Army."
Activists have posted a video they say shows the bodies of a number of men and boys killed in Tremseh.
However, state media provided a very different version of events.
The Sana news agency quoted military sources as saying terrorist groups had been using Tremseh as a "springboard for their criminal acts".
The army launched a "qualitative operation" after the village's inhabitants had pleaded with them to intervene.
The Sana report said after the "terrorists" were killed, soldiers found the bodies of civilians who had been abducted and killed by the terrorists.
Reports of casualties often cannot be independently verified, as Syria severely restricts journalists' freedom of movement.
Some 16,000 people are thought to have been killed since the uprising against Bashar al-Assad's regime began in March 2011.
Meanwhile, the Wall Street Journal on Thursday quoted intelligence reports as suggesting that Syria was moving its chemical weapons, amid fears the government could use them against rebels or civilians.