UN and Arab League envoy for Syria Kofi Annan has said he is “shocked and appalled” at reports of mass killings in the Syrian village of Tremseh.
Activists say the army and pro-government militiamen killed 200 people in the village on Thursday. The Syrian government blamed “terrorist groups”.
Mr Annan accused the government of violating his six-point peace plan.
If the death toll in Tremseh is confirmed, it would be the bloodiest single event in the Syrian conflict.
Residents of the village, in Hama province, told activists it was attacked with helicopter gunships and tanks, and later by the pro-government Shabiha militia, who carried out execution-style killings.
The Sana news agency gave a different account on Friday, saying: “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.”
The mandate for the UN’s observer mission to Syria expires on 20 July and council members are locked in debate about a new resolution, which must be passed before the deadline if the mission is to be renewed.
Reacting to the events in Tremseh, Mr Annan said: “I am shocked and appalled by news coming out of the village of Tremseh, near Hama, of intense fighting, significant casualties, and the confirmed use of heavy weaponry such as artillery, tanks and helicopters.
“This is in violation of the government’s undertaking to cease the use of heavy weapons in population centres and its commitment to the six-point plan.”
Mr Annan said UN observers were ready to travel to Tremseh to investigate the killings.
The head of the UN Supervision Mission in Syria (Unsmis), Maj Gen Robert Mood, said this could only take place “when there is a credible ceasefire”.
UK Foreign Secretary William Hague said UN observers must be allowed access to Tremseh “quickly and without hindrance so they can carry out an independent investigation”.
He said: “The time has come for decisive diplomatic action to bring about a peaceful end to the violence.”
France also called for renewed UN action. Foreign ministry spokesman Bernard Valero said: “Stiffened resolve must now be shown with the threat of sanctions from the Security Council. The time has come for everyone to assume their responsibilities.”
Mr Annan added: “It is desperately urgent that this violence and brutality stops and more important than ever that governments with influence exert it more effectively to ensure that the violence ends – immediately.”
Mr Annan is to travel to Moscow early next week for talks on the Syria crisis.
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.
State media said gunmen from what they termed armed terrorist groups had attacked Tremseh in the morning, shooting dead dozens of people.
Individual 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.
UN Security Council envoys have been meeting in New York to try to hammer out a resolution ahead of the expiry of the Syrian mission’s mandate.
Western nations are pressing the UN to threaten Syria with more sanctions, a move Russia and China oppose.
The UN’s mission had a 90-day remit to monitor a truce, but fighting has continued largely unabated.
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.