Some 200 people have been killed in an attack on the Syrian village of Tremseh, opposition activists say.
If confirmed, it would be the bloodiest single event in the Syrian conflict.
Residents said the village, in Hama province, was attacked with helicopter gunships and tanks, and later by the pro-government Shabiha militia, who carried out execution-style killings.
State media blamed "terrorist groups" who were trying to raise tension ahead of a key UN Security Council meeting.
The mandate for the UN's observer mission to Syria expires on 20 July.
UN observers are now trying to get to Tremseh to investigate the killings.
UN and Arab League special envoy for Syria Kofi Annan said he was "shocked and appalled" by the news from Tremseh, adding that it was "desperately urgent that this violence and brutality stops".
Some 16,000 people are thought to have been killed since the uprising against Bashar al-Assad's regime began in March 2011.
Individual reports of casualties often cannot be independently verified, as Syria severely restricts journalists' freedom of movement.
Reports suggest the army was trying to take back Tremseh after it had fallen into rebel hands.
The BBC's Jim Muir in Beirut says both sides agreed many people were killed in Tremseh, but have totally different versions of what happened.
Activists say government forces surrounded the village on Thursday morning and heavily bombarded it for several hours, killing many people.
Pro-government militias from nearby Alawite villages then moved in, they said, killing many more villagers and setting fire to houses. Others who tried to flee through fields were also gunned down, the activists said.
One activist, named Ahmed, told Reuters: "So far, we have 20 victims recorded with names and 60 bodies at a mosque. There are more bodies in the fields, bodies in the rivers and in houses... People were trying to flee from the time the shelling started and whole families were killed trying to escape."
Activists have posted a video they say shows the bodies of a number of men and boys killed in Tremseh.
Our correspondent says UN observers are talking to both sides in the conflict so that they can move from their base in Hama to Tremseh to investigate the killings.
The head of the UN Supervision Mission in Syria (Unsmis), Maj Gen Robert Mood, said: "Unsmis stands ready to go in and seek verification of facts if and when there is a credible ceasefire."
Protests condemning the attack have been reported in Damascus, Idlib and Hama.
State media said gunmen from what they termed armed terrorist groups had attacked the village in the morning, shooting dead dozens of people.
The Sana news agency said on Friday: "The bloodthirsty media in collaboration with gangs of armed terrorists massacred residents of Tremseh village... to sway public opinion against Syria and its people and provoke international intervention on the eve of a UN Security Council meeting."
Western nations are pressing the UN to threaten Syria with sanctions as it considers renewing the mandate for its observer mission in Syria.
They want a 10-day ultimatum to be part of a Security Council resolution on the future of the mission. A new resolution must be passed before the mission's mandate ends next Friday.
China and Russia continue to oppose any moves to threaten Damascus with further sanctions ahead of the 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 Mr Annan, who has called for "clear consequences" for the Syrian government and rebels if the ceasefire is not observed.
Mr Annan is to travel to Moscow early next week for talks on the Syria crisis, Russian media say.
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.
US state department spokeswoman Victoria Nuland said the international community would hold accountable any Syrian officials who failed to safeguard the stockpiles.
BBC diplomatic correspondent Jonathan Marcus says the Syrian transfer could be an attempt at safe storage or may mark an even more deadly phase in the conflict.