Syrian government forces have recaptured over a quarter of the rebel-held enclave of Eastern Ghouta, bringing them close to the main town of Douma, a monitoring group said on Sunday.
It comes amid a two-week-long bombing campaign against the Damascus suburb that has killed hundreds of civilians and sent hundreds more fleeing to western parts of the enclave.
The Britain-based Syrian Observatory for Human Rights said government forces had advanced to within three kilometres of Douma after retaking "more than 25 per cent" of Eastern Ghouta, Agence France-Presse reported. Operations have so far taken government forces mostly through farmlands.
Fifteen days of air strikes, artillery fire and rocket attacks are reported to have killed more than 640 civilians, according to Agence France-Presse.
The United Nations said on Sunday that it plans to deliver much-needed humanitarian assistance to Eastern Ghouta residents on Monday.
A convoy consisting of "46 truckloads of health and nutrition supplies, along with food for 27,500 people in need" will enter the enclave, said the UN Office for the Co-ordination of Humanitarian Affairs (Ocha).
Eastern Ghouta's 400,000 residents have lived under regime siege since 2013, facing severe shortages in food and medicine.
"We hope that the convoy may proceed as planned and will be followed by other convoys," said Ali Al Zaatari, the UN's humanitarian coordinator in Syria who will be leading Monday's operation.
"Our teams on the ground are ready to do all that is needed to make this happen."
Russia, the main ally of Syrian President Bashar Al Assad, last week announced daily five-hour "humanitarian pauses" in Eastern Ghouta. But although the air campaign has eased, fighting has intensified on the ground.
With the support of Russian warplanes, the Syrian military has advanced on several fronts, retaking control of farms and villages, a military source told state media.
The source said government forces had seized a number of districts including Al Nashabiyeh and Otaya, and had "eradicated terrorist groups" on the eastern outskirts of Damascus. The government refers to all armed opposition groups as "terrorist".
The Observatory meanwhile, said Assad's forces had reached the centre of Eastern Ghouta, to the edge of the town of Beit Sawa.
Hundreds of civilians were on Sunday fleeing Beit Sawa.
"Everyone is on the road. There's destruction everywhere," said 35-year-old Abu Khalil, carrying a little girl in his arms who was wounded on the cheek.
The only UN delivery of aid to Eastern Ghouta so far this year took place on February 14 when a convoy with assistance for 7,200 people reached Al Nashabiyeh.
OCHA said on Sunday that a new delivery of aid would be taken in to Eastern Ghouta by the UN and its partners, after they "received approval to deliver assistance for 70,000 people in need in the besieged enclave".
"The UN has received assurances that the remaining supplies for all approved people in need will be delivered on March 8, 2018," it added.
"The United Nations calls on all parties to facilitate unconditional, unimpeded, and sustained access to all people in need throughout the country."
Although opposition fighters are on the back foot in Eastern Ghouta they are not going without a fight and have even retaken some parts of the area of Shifoniya, according to the Observatory.
The monitor's head, Rami Abdel Rahman, said at least 12 regime fighters had been killed in Shifoniya and another area, Al Rihan, in overnight clashes with the Jaish Al Islam opposition group.
Jaish Al Islam shares control of rbeel-held parts of Eastern Ghouta with Faylaq Al Rahman and Ahrar Al Sham.
Hamza Bayraqdar, a spokesman for Jaish Al Islam, said on Twitter that the group's forces had launched "surprise attacks" against regime positions.
Encircled by regime-controlled territory and unable or unwilling to flee, Eastern Ghouta's 400,000 residents have in recent weeks suffered one of the most ferocious assaults of the Syrian war.
According to the Observatory, some 2,000 civilians have now fled regime shelling and clashes in eastern areas to reach western parts of the enclave.