The Syrian Observatory for Human Rights said government forces shelled Deir Qanun, a village in the Wadi Barada region, which is the main source of water for Damascus.
"This is the highest toll there since the beginning of the truce" on December 30, Observatory head Rami Abdel Rahman told AFP.
At least 20 other people were wounded in the attack, some critically.
Activists from the Wadi Barada Media Committee said on Twitter that the shells struck a temporary shelter where displaced women and children had been seeking refuge.
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Ahmed al-Ghadban had been on his way to the main Ain al-Fijeh spring with government maintenance teams when he was killed.
Opposition fighters and government officials accused each other of killing the retired army officer, who had only assumed his duties yesterday.
Under the agreement, Ghadban was to oversee teams working to repair the infrastructure that supplies the capital with water in exchange for a cessation of hostilities and the withdrawal of rebel fighters willing to do so.
Fighting has persisted in Wadi Barada since the entry into force on December 30 of the ceasefire brokered by Russia and Turkey.
The ceasefire and planned talks are the latest effort to negotiate an end to a conflict that has killed more than 310,000 people since it began with anti-government protests in March 2011.
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