The Syrian Observatory for Human Rights said overnight air strikes killed at least eight people in the rebel-held Eastern Ghouta.
The deaths were the first civilian casualties in the area - the last major rebel stronghold near Damascus - since a cessation of hostilities was declared there on Saturday. The strikes, targeting the town of Arbin, wounded an additional 30 people, it said.
The Civil Defence for rural Damascus, a rescue service operating in the area, said the dead included five children and two women.
In a statement on its Facebook page, it put the number of wounded and missing at 50. The air strikes hit the area at 11 p.m. (2000 GMT), it said.
There was no immediate comment from the Syrian military on the report, and no mention of air strikes by state media.
The Observatory said: "This is the first time that civilian martyrs have fallen as a result of regime or warplane bombardment since the start of the ceasefire agreement in the Eastern Ghouta".
Russia, a military ally of President Bashar al-Assad, said it had deployed military police in the Eastern Ghouta on Monday to try to enforce a de-escalation zone it said it had agreed with the Syrian opposition there.
The Syrian military declared a cessation of hostilities there on Saturday.
The Syrian army, with military support from Russia and Iran, has dealt the opposition a string of defeats around the capital over the last year, seizing back control of areas including Daraya and Moadamiya.
