"More than 700 fighters from the Syrian government's allied militias took new positions on the Eastern Ghouta fronts", the Britain-based Syrian Observatory for Human Rights said.
The United Nations says an aid convoy carrying humanitarian assistance has started entering the town of Douma in Damascus' besieged rebel-held suburbs of eastern Ghouta.
He says that "nearly half of the food carried on the convoy" could not be delivered.
More than one-third of Eastern Ghouta has fallen under government control since Damascus launched a ground offensive against the opposition enclave in recent weeks.
A military source, speaking on condition of anonymity, said Syrian army troopers had captured farms southeast of the towns of Rihan and Housh al-Ashary.
"People were afraid", he said, "They didn't want government troops to reach their doorsteps and kill them and their families", Khouli said.
Eastern Ghouta has been under siege for the last five years, and humanitarian access to the area, which is home to some 400,000 people, has been completely cut off.
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According to the Observatory 805 civilians, among them 178 children, were killed in Eastern Ghouta since February 18.
Since Feb. 19, at least 834 people have been killed in Eastern Ghouta, and the death toll is expected to rise further as attacks by the regime remain ongoing.
The ICRC confirmed that its joint convoy with the United Nations had to leave before offloading all its supplies on account of the deteriorating security situation.
In their 15th report, published Tuesday, the investigators also accused a US-led coalition of breaching worldwide humanitarian law with a deadly strike a year ago on a school near the Islamic State group's former stronghold of Raqa. "The people sit with multiple families, with thousands of families in basement shelters", he said.
An global aid convoy to the besieged Syrian opposition enclave of Eastern Ghouta has been forced to cut short its mission due to heavy bombardments by the Syrian regime.
A boy stands on the rubble of a damaged building at the besieged town of Douma, Eastern Ghouta, Damascus, Syria March 5, 2018.
Iyad Abdelaziz, a member of the Douma Local Council, says nine aid trucks had to leave the area after government shelling and airstrikes intensified in the evening.
The brutal onslaught on the last major rebel stronghold near the capital, the latest regime offensive of the seven-year civil war, has sparked outrage but little action from the West.