Pro-government militiamen executed at least 16 civilians in their homes in Damascus before dawn on Wednesday, the Syrian Observatory for Human Rights said.
"Militiamen burst into their homes in the Barzeh neighbourhood at 5 am (0200 GMT) and shot them dead," Observatory chief Rami Abdel Rahman told AFP.
He said those killed included six women and three children.
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Activist network the General Committee of the Syrian Revolution said that a father and his three children, a mother and her son, and a couple and their daughter were among the dead.
Barzeh is a Sunni Muslim neighbourhood of north Damascus considered sympathetic to the opposition to the minority Alawite-dominated regime of President Bashar al-Assad.
Mainly Alawite militia groups have been accused of carrying out some of the most notorious killings of civilians during the 18-month uprising against Assad's rule.