Syrian government forces have massed around the last rebel-held town in the Eastern Ghouta area, after rebels were reportedly given a deadline to leave.
A newspaper said troops were preparing for a “huge” operation in Douma, which is controlled by Jaysh al-Islam.
The government’s ally Russia is said to have told the group late on Monday it had 48 hours to agree to be evacuated to Idlib province like other factions.
But Jaysh al-Islam has said its fighters want to disarm and stay.
The United Nations is concerned about the fate of the estimated 70,000 civilians trapped in Douma, where fighting has continued despite the talks with Russia.
More than 1,700 people are reported to have been killed and thousands injured since the government and its allies launched an offensive on 18 February to retake the Eastern Ghouta, which was the last major rebel stronghold near Damascus.
The UN says 80,000 civilians have fled on foot to government-held territory on the edge of the capital, as troops made dramatic advances in recent weeks and split the region into three pockets – the largest of which was around Douma.
Almost 20,000 rebels and civilians have also been evacuated to Idlib under deals between the government and the rebel factions Ahrar al-Sham and Faylaq al-Rahman, which controlled the pockets around the towns of Harasta and Arbin.