The Syrian Observatory for Human Rights said 30 people, all of them men ,had been killed in the operation, backed by fighters of Lebanon's Hezbollah.
But it said it could not confirm whether all of the dead were rebels.
A Hezbollah source confirmed that the group's fighters had taken part in the operation and said 30 rebels had been killed.
The Shiite militant group has deployed thousands of fighters to Syria to support President Bashar al-Assad's regime against the mainly Sunni rebels.
Countryside east of Damascus known as Eastern Ghouta has remained largely in rebel hands, despite repeated efforts by the army to oust them.
In August 2013, the area was hit by a string of chemical weapons attacks that killed hundreds of people.
The attacks sparked US threats of military action that were defused only by the government's agreement to dismantle its chemical arsenal.
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But it said it could not confirm whether all of the dead were rebels.
A Hezbollah source confirmed that the group's fighters had taken part in the operation and said 30 rebels had been killed.
The Shiite militant group has deployed thousands of fighters to Syria to support President Bashar al-Assad's regime against the mainly Sunni rebels.
Countryside east of Damascus known as Eastern Ghouta has remained largely in rebel hands, despite repeated efforts by the army to oust them.
In August 2013, the area was hit by a string of chemical weapons attacks that killed hundreds of people.
The attacks sparked US threats of military action that were defused only by the government's agreement to dismantle its chemical arsenal.
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