The Revolution Leadership Council of Hama said Tremseh was first shelled, then invaded by pro-government militiamen of Assad's Alawite sect, who swept in and killed victims one by one. It said some civilians were killed while trying to flee.
Multiple reports quoting activists and witnesses said a convoy of vehicles from nearby Alawite villages surrounded Tremseh early Thursday. Militiamen then blockaded the settlement and began randomly firing on houses as a helicopter flew overhead. Electricity and telephone lines were cut. Bodies were later recovered from fields, private homes and the nearby Orontes River.
The previous largest massacre in Syria took place in nearby Houla, when 108 men, women and children were killed on May 25 - the vast majority of them executed, according to a United Nations report. In June, it took U.N. monitors two days to reach the site of an alleged massacre of 78 people shot, stabbed or burned alive in the Sunni hamlet of Mazraat al-Qubeir by pro-government shabiha militiamen.
Most Assad loyalists - including the shabiha - are minority Alawites, who form a branch of Shi'ite Islam. The street protesters and fighters behind the uprising are mostly Sunni Muslims.
Activist groups have put the death toll in the 16-month-old conflict at more than 17,000 people.
US concerns about chemical weapons
Meanwhile, the United States says the international community will hold Syrian officials accountable if they fail to meet their duty to safeguard the country's stockpiles of chemical weapons.
State Department spokeswoman Victoria Nuland says the U.S. has repeatedly warned Syria it is obligated to protect those weapons.
Nuland made her comments after a prominent U.S. newspaper reported that Syria has started moving part of its chemical weapons arsenal out of storage facilities.
The Wall Street Journal quoted U.S. officials as saying Syria may use the weapons against rebels or civilians.
Syria is believed to have reserves of sarin nerve agent, mustard gas and cyanide.
Throughout the conflict in Syria, the newspaper said, U.S. officials and their allies in the region have been watching for changes in the status or location of Syria's alleged chemical weapons.
VOA's Snowiss reported from Washington and Yeranian from Cairo. Some information came from AP, AFP and Reuters.