Kofi Annan says he is "appalled" by the massacre of up to 200 people in the Syrian village of Tremseh, one of the bloodiest incidents in the country's ongoing conflict.
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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.
However, Reuters says it has seen a UN observers' report which attributes the atrocity to "an extension of the SAAF (Syrian Arab Air Force) operation".
"The situation in Hama province continues to be highly volatile and unpredictable," the so-called 'flash report' from the observer mission said.
"SAAF forces continue to target populated urban areas north of Hama City in a large scale."
Kofi Annan, the UN and Arab League envoy for Syria, 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."
If it is confirmed, the Tremseh attack would be one of the bloodiest single events in the Syria conflict.
Speaking in the Syrian capital Damascus, Major General Robert Mood, the head of UNSMIS (the United Nations mission) said that the UN could verify that there had been continuous fighting in the Tremseh region on Thursday, involving mechanical units, indirect fire and helicopters. He added:
"UNSMIS stands ready to go in and seek verifications of facts if and when there is a credible ceasefire."
Some 16,000 people are thought to have been killed since the uprising against Bashar al-Assad's regime began in March 2011.
Foreign Secretary William Hague said: "The reports of a massacre in the town of Traymseh in Hama district are shocking and appalling.
"More than 200 men, women and children appear to have been killed in this latest atrocity.
"Everything we have seen of the Syrian regime's behaviour over the last seventeen months suggests that these reports are credible. They demand a united response from the international community.
"We have two urgent priorities – to establish an accurate account of what happened in Traymseh so that those responsible can and will be held to account; and to agree urgent action at the United Nations Security Council. "
Dead 'were civilians'
Reports suggest the army was trying to take back Tremseh after it had fallen into rebel hands.
The Revolution Leadership Council of Hama said 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 Mr 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.
More rebel weapons
Partly as a result of more weapons, the rebels' capabilities have improved in recent months and they have inflicted rising numbers of casualties on the regular army.
According to the UK-based Syrian Observatory for Human Rights, at least 4,348 members of security forces loyal to the regime have died since the start of the uprising in March 2011. The conflict has also claimed the lives of 11,897 civilians and armed insurgents and 884 defecting soldiers.
However, 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.
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.