More than 200 people have been killed by forces loyal to President Bashar al-Assad in the village of Tremseh in Hama province, according to opposition activists.

The Revolution Leadership Council of Hama told Reuters that most of those killed were civilians. The opposition activists said the village had been attacked by helicopter gunships and tanks, and that the pro-government Shabbiha militia then moved in and shot more people in execution-style killings.

Syrian state television said three security personnel had been killed in fighting in Tremseh, and accused "armed terrorist groups" of committing a massacre in the village.

A statement by the Hama Revolutionary Council said: "More than 220 people fell today in Tremseh. They died from bombardment by tanks and helicopters, artillery shelling and summary executions."

Fadi Sameh, an opposition activist from Tremseh, said he had left the town before the reported massacre but was in touch with residents.

"It appears that Alawite militiamen from surrounding villages descended on Tremseh after its rebel defenders pulled out, and started killing the people. Whole houses have been destroyed and burned from the shelling.

"Every family in the town seems to have members killed. We have names of men, women and children from countless families."

He added that the bodies of many of those killed were taken to a local mosque.

Ahmed, an activist from the Union of Hama Revolutionaries said: "We have reports of more than 220 killed. 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."

The reports could not be independently confirmed, but if they are accurate, the incident would be the worst massacre since the uprising against Assad's regime began 16 months ago.

Reports of the massacre come as the international community remains deadlocked over how to resolve the Syrian crisis, with Russia and China continuing to oppose a western move to press the United Nations to threaten Damascus with sanctions before 20 July.

That is the day the mandate expires for the current UN mission which, despite a 90-day remit, has failed to stem a mounting casualty toll in Syria, where the daily death toll often tops 100.

China and Syria's allies have agreed to an extension of the mission, but have refused to support a UN resolution that allows for the threat of new sanctions.

Western nations want to ensure that a 10-day ultimatum to Syria to withdraw its heavy weapons and pull back from towns and cities is a central part of a new resolution, which must be passed before the current mission's term expires next Friday.

UN and Arab League envoy Kofi Annan has warned of "clear consequences" if a ceasefire does not take root this time.

Annan has returned from visits to Damascus, Tehran and Baghdad, where he has tried to secure support for a new attempt to establish a ceasefire in Syria.

• This article was amended on 13 July. The death toll was revised from "more than 250" to "more than 200".