The Britain-based Syrian Observatory for Human Rights said Saturday that more than 90 people had been killed in the Houla area, in the province of Homs, in the last 24 hours.
A local activist giving his name as Abu Yazan reached via Skype said 12 people died in shelling and 106 were killed when pro-regime thugs known as shabiha stormed the area.
That death toll is one of the highest for any single event since the popular uprising against Bashar Assad began in March 2011. The UN says more than 9,000 people have been killed, most of them civilians.
The UN said Saturday a team of observers was heading to the area.
Fighting in Syria has continued despite the deployment of some 250 UN observers who have fanned out across the country to monitor a ceasefire brokered by UN envoy Kofi Annan.