Around 150 people have been killed Thursday (July 12th) in the Syrian village of Treimsa in what the opposition said was a massacre by government forces, AFP reported.

The Syrian Observatory for Human Rights on Friday said regime troops attacked the village with tanks and helicopter gunships and were backed by militias armed with guns and knives.

"Some are estimating higher numbers, but even at around 150, especially considering how small the town is, this might be the biggest massacre committed in Syria since the start of the revolution," the Observatory's head Rami Abdel Rahman told AFP.

The Syrian government initially blamed "terrorists" for perpetrating the attack. However, the official Syrian news agency SANA later said regime troops killed a large number of "terrorists" during an operation that did not lead to any civilian casualties.

Kofi Annan, the UN special envoy to Syria, said in a statement Friday that he was "shocked and appalled" by the attack, and "the confirmed use of heavy weaponry such as artillery, tanks and helicopters".

He added, "I condemn these atrocities in the strongest possible terms" and that it was "desperately urgent that this violence and brutality stops".

EU foreign policy chief Catherine Ashton denounced the massacre as "a blatant violation" of the plan brokered by UN-Arab League envoy Kofi Annan.

"I am deeply shocked by reports of the ruthless killing of at least 200 men, women and children in the village of Tremseh in the Hama region," she said in a statement. "The regime's use of heavy weaponry, including artillery and helicopters, which has been confirmed by UN observers, is a blatant violation of its obligations under the Annan plan."

Meanwhile, France called for "stiffened resolve... with the threat of sanctions from the Security Council."

"This tragedy shows how much the first step towards a cessation of violence must be taken by the Syrian government," foreign ministry spokesman Bernard Valero told journalists in Paris.

British Foreign Secretary William Hague called for an investigation to establish what occurred in Treimsa and for "urgent action" at the Security Council.