At least 200 people are reported to have been killed in the Syrian village of Tremseh, in Hama province.
Opposition activists quoted residents as saying the village was attacked with helicopter gunships and tanks.
Pro-government Shabiha militia later went in on foot and carried out execution-style killings, they said.
State media said “terrorist groups” had carried out a massacre to raise tensions ahead of a UN Security Council meeting on the Syria observer mission.
If it is confirmed, the Tremseh attack would be one of the bloodiest single events in the Syria conflict.
Some 16,000 people are thought to have been killed since the uprising against Bashar al-Assad’s regime began in March 2011.
Reports suggest the army was trying to take back Tremseh after it had fallen into rebel hands.
The Revolution Leadership Council of Hama told the Reuters news agency that most of the dead in Tremseh were civilians. Protests condemning the attack have been reported in Damascus, Idlib and Hama.
Earlier on Thursday, Syria’s ambassador to Iraq Nawaf Fares announced his defection, following in the footsteps of a former senior general who escaped the country earlier this week.
Iraqi officials have said Fares, who has publicly declared his support for the opposition, is in Qatar.
Syrian forces also shelled the suburbs of Damascus later in the day in an apparent offensive against rebel fighters.
Western nations are pressing the UN to threaten Syria with sanctions as it considers renewing the mandate for its observer mission in Syria which expires on 20 July.
They want a 10-day ultimatum to be part of a Security Council resolution on the future of the UN’s observer mission in the country. A new resolution must be passed before the mission’s mandate ends on Friday next week.
China and Russia continue to oppose any moves to threaten Damascus with further sanctions ahead of the 20 July deadline.
The mission had a 90-day remit to monitor a truce, but fighting has continued largely unabated.
The truce formed part of a six-point peace plan brokered by UN-Arab League envoy Kofi Annan, who has called for “clear consequences” for the Syrian government and rebels if the ceasefire is not observed.