The activists and witnesses say troops shelled the village of Treimsa with tanks and helicopters before storming the area. A rebel leader put the death toll at more than 200.
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.
Thursday’s reported massacre comes as the United Nations Security Council debates imposing tough new sanctions on the Syrian government if it keeps attacking the opposition and civilians with heavy weapons.
The Western-backed resolution also leaves the door open to possible military intervention. But Russia is threatening to veto the resolution. It wants to simply extend the mandate of the U.N. observer mission in Syria for another three months.
Moscow has been one of Syria’s closest allies and has vetoed previous U.N. action.
The Syrian ambassador to Iraq, Nawaf Fares, who has defected to join the opposition, is urging Syrian soldiers to join him. He says there is no honor in killing one’s own people.
Fares is the highest-ranking Syrian official to switch sides since the uprising against President Bashar al-Assad began 16 months ago.