Syrian government and allied forces have retaken a large area south-east of Damascus from rebels, including the key town of Deir al-Asafir, activists say.
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 Syrian Observatory said pro-government forces had launched "a violent attack and heavy air campaign" as the rebels moved to shore up the frontline.
Soldiers, backed by fighters from the Lebanon's Hezbollah movement, took advantage of weeks of infighting among rebel groups to advance, it added.
A powerful Islamist rebel group, Jaish al-Islam, reportedly pulled 800 of its fighters from the area in recent days, leaving remaining rebels scrambling to fill the void.
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.
Pro-Hezbollah TV channel Al Manar said the assault was part of a new military operation by the Syrian army against the rebels.
More than 270,000 people have been killed in five years of civil war in Syria.