Nine bodies, some of them mutilated, were found in a town near Damascus, the Syrian Observatory for Human Rights said on Friday.
The Britain-based watchdog said it was unclear who carried out the killings but opposition groups blamed pro-government militia and demanded international action to prevent such "massacres."
"The bodies of nine people were found in Hamouria in Damascus province, some with their throats slit," the Observatory said.
"It is unclear who carried out the attack," the watchdog's head Rami Abdel Rahman told AFP.
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It said the killers had "cut off the hands, legs and genitalia" of the dead.
The Revolutionary Command Council in Damascus province said that the killings came amid an intense bombardment of the area by government forces, using heavy artillery, tanks and aircraft.
The activist group called on Arab and Islamic countries, and the United Nations to "put an end to the crimes of the regime and the massacres of its militias."
In all, at least 84 people died in clashes and bombings across Syria on Thursday, 48 of them civilians, the Observatory said.
Activists called for nationwide protests on Friday as the uprising against President Bashar al-Assad's regime enters its 16th month.
More than 14,400 have been killed since the revolt began in March last year, according to the Observatory's figures.