Syrian opposition activists say government forces with tanks and helicopters on Thursday killed more than 150 people, mostly civilians, in a village in Hama province.

The activists and witnesses say troops shelled the village of Treimsa Thursday before storming the area. Estimates of the number of deaths are as high as 220.

State-run Syrian media say troops battled an armed group in the area and, as in previous mass killings, blamed the massacre on those it calls terrorists.

International envoy Kofi Annan said in a statement Friday he is “shocked and appalled” by the reports and condemns the violence in the strongest possible terms.

The reported massacre came as the U.N. Security Council debates imposing tough new sanctions on the Syrian government if it keeps attacking civilians with heavy weapons.

Russia is threatening to veto the Western-backed resolution. It wants to simply extend the mandate of the U.N. observer mission in Syria for another three months.

Russian media say Annan will visit Moscow on Monday for talks on Syria.