Syrian regime troops have retaken a large area south-east of Damascus from rebels, including the key town of Deir al Asafir, the BBC cited activists as saying.
The town and nine surrounding villages, which had been controlled by rebels since 2012, reportedly fell in a matter of hours.
Rebel groups in the area had recently clashed with one another.
Large numbers of civilians from the Eastern Ghouta agricultural belt are said to be fleeing and heading north.
The Syrian Observatory for Human Rights, a UK-based monitoring group, and the Local Co-ordination Committees, an opposition activist network, said pro-government forces had seized the rebel-held areas on Thursday morning.
The Eastern Ghouta was included in a "regime of calm" declared by the government late last month, but there had been mounting violence there.
The government had been trying to take Deir al Asafir and the surrounding area since February, according to Syrian Observatory director Rami Abdul Rahman.
More than 270,000 people have been killed in five years of civil war in Syria.