Syrian opposition activists said more than 200 people were killed in a Sunni village on Thursday by government forces using tanks and helicopters, which, if confirmed, would be the worst in a series of massacres that have convulsed Syria’s increasingly sectarian uprising against President Bashar al-Assad.

The Syrian government also reported a mass killing in the village but said it was committed by armed terrorist groups, the official description for Mr. Assad’s opponents. It said at least 50 people were killed.

The site of the reported massacre, the village of Tremseh near the city of Hama, a focal point of the 17-month-old uprising, was the first mass killing since United Nations cease-fire monitors suspended their work in Syria a month ago because conditions were too dangerous.

Activists in Hama posted a video on YouTube accusing the government of “ethnic cleansing in Hama,” and said the killings in Tremseh were “unlike any massacre that has previously occurred in Syria.” Tremseh is a Sunni village surrounded by villages populated by Mr. Assad’s Alawite sect.