The Britain-based Syrian Observatory for Human Rights said the casualties were caused by a series of airstrikes that struck the rebel-held town of Deir al-Asafir, which lies east of Damascus in an area known as Eastern Ghouta.
Four children and a civil defense worker were among the victims, the Observatory said. The Local Coordination Committees, another opposition activist group, put the death toll from the airstrikes at 17.
The Western-backed Syrian opposition says the government of President Bashar Assad has been targeting civilians despite the truce.
The Syrian National Coalition, an opposition group, denounced the "massacre" in Deir al-Asafir, saying it threatened to derail the cease-fire and peace talks that are scheduled to resume in Geneva in two weeks.
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The Observatory says around 2,700 families live in Deir al-Asafir. Government troops have been trying to encircle the town for weeks.
There was no immediate reaction from the Syrian government, which rarely comments on security issues.
The talks in Geneva are meant to start a political process for transition in Syria away from Assad. In comments made in an interview with Russia's state news agency Sputnik, however, Assad rejected a key opposition demand for a transitional ruling body with full powers, which major powers agreed on at a Geneva conference in June 2012.