Bashar al-Assad’s troops have made major gains in Eastern Ghouta, splitting the enclave in three, as a group of rebels left in the first evacuation deal.
The advancements separate the largest town of Douma from the rest of the opposition pocket and trapping civilians in increasingly shrinking territory.
As part of a divide-and-conquer strategy, Syrian troops seized control of the road linking Douma with the town of Harasta further west, and also captured the town of Misraba.
The government now controls more of Eastern Ghouta - an estimated 40 per cent - than at any time since they besieged the area five years ago.
Jaish al-Islam, one of the dominant rebel groups in the Damascus suburb, agreed to evacuate prisoners from an Islamist rebel group aligned to al-Qaeda that they had been holding.
The extremist militants from Hayat Tahrir al-Sham (HTS) are excluded from the yet-to-be-implemented UN ceasefire and their presence provided a justification for the government bombardment of the area.
Eastern Ghouta is controlled by a complex patchwork of rebels including Jaish al-Islam and Failaq al-Rahman, which have distanced themselves from HTS.
The some 13 fighters and their families were sent to a town in central Hama province controlled by HTS.
A humanitarian corridor was set up for civilians by the Syrian government and its Russian ally last week, but so far few have left.
A UN official who accompanied a recent relief convoy into the pocket said both the regime and the rebels were preventing civilians from leaving.
Sajjad Malik, the UN Refugee Agency’s representative to Syria, said snipers from opposition groups positioned near the corridor, and government air strikes, were stopping people escaping the escalating violence.
“They said, ‘These guys are preventing us’ ” while pointing at nearby rebel fighters, Mr Malik said.
“They want out — either the bombing to stop, or to get out. But reach safety where?” he said in an interview in Beirut. “We are getting to a point where there is literally no flight option. What worse situation could there be?”
The UN convoy was the first to reach the some 400,000 residents of Eastern Ghouta since the government began its offensive there in mid-February.
Families have been living in their basement to avoid the air strikes, leaving only to try to find food.
Some 1,000 have been killed and as many as 3,000 injured in the last three weeks.
“I’ve never seen such scared faces in my life that I’ve seen there,” Mr Malik added. “You can see it in their eyes, you can see it in their expressions. But they’re also desperate for someone to come and help them out.”
He said the town was marked by the stench of death, with bodies lying unclaimed in the rubble.