Rights activists have accused Syrian security forces of massacring as many as 200 people in the central Hama province, The Guardian reported.
Witnesses said troops used tanks and helicopter gunships Thursday in an assault on residents in the village of Tremseh.
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UN-Arab League special envoy Kofi Annan, who met the Syrian President for talks on Monday, said the confirmed use of such equipment was in breach of the regime's commitment to his six-point peace plan.
"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," Annan said.
Rebel leader Abu Mohamad told Agence France Presse that Syrian forces were helped by fighters from the pro-regime Shabiha militia, said to accompany troops to make sure they do not desert.
Mohamad said Tremseh was bombarded from 11am to 9pm Thursday. The BBC cited reports that said Syrian forces were looking to take back Tremseh after it had fallen into rebel hands.
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Syrian state media blamed the massacre on what it called "terrorist groups," adding the violence had been timed to raise tensions ahead of a UN Security Council meeting on the Syria observer mission, whose mandate is due to end next week.
UN observers released a "flash report" on Friday, saying that the Syrian Arab Air Force continued to target populated urban areas north of Hama, according to Reuters. The report said, "The operation in Tremseh is assessed as an extension of the SAAF operation in Khan Sheikhoun to Souran over the recent number of days."
The report said unarmed UN military observers were stopped by SAAF commanders but could still observe at least 100 explosions, as well as small arms and heavy machine gun fire from their positions at different locations around Tremseh. The observers also saw an Mi-8 and two Mi-24 helicopters flying overhead, according to Reuters.
The UN Security Council is holding talks on rival draft resolutions by Russia and Western nations on renewing the UN mission in Syria, which was suspended last month amid an increased risk of violence.
The opposition Syrian National Council (SNC) this morning demanded a binding UN resolution following the Tremseh massacre, AFP reported.
"To stop this bloody madness which threatens the entity of Syria, as well as peace and the security in the region and in the world, requires an urgent and sharp resolution of the Security Council under Chapter VII (of the UN Charter) which protects the Syrian people," the SNC said.
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Russia has continued to reject the imposition of sanctions against Syria, and says it will veto the Western draft if it is brought to a vote.
According to Russian news agencies, Annan will meet Russian Foreign Minister Sergei Lavrov on Monday for fresh talks on the Syrian conflict.