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.
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.
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.
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.
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.
"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.
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.