Syrian troops on Saturday cut off the largest town in Eastern Ghouta from the rest of the rebel-held enclave which is now split in three, a monitor said.
The army captured a road linking the town of Douma with Harasta further west, and also captured the town of Misraba, the Syrian Observatory for Human Rights said.
"Regime forces have therefore divided Eastern Ghouta into three parts -- Douma and its surroundings, Harasta in the west, and the rest of the towns further south," the Britain-based war monitor said.
An AFP correspondent inside Douma said shelling and air strikes were rocking the town on Saturday.
Besieged by government forces since 2013, Ghouta is the last remaining rebel-held area near the capital.
Keen to secure Damascus, government troops and allied militia have been closing in on the enclave since February 18 and have overrun more than half of it.
More than 975 civilians, over 250 of them children, have been killed and hundreds more wounded, according to the Observatory.
Two main Islamist rebel groups are present in the enclave, but jihadists led by Al-Qaeda's former Syria affiliate are also active.