Russian Federation said on Saturday that it was negotiating with rebels to let them leave Eastern Ghouta safely if they surrendered the enclave to government forces.
After the army advances split up the enclave, Jaish al-Islam emerged as the strongest group in the town of Douma, Ahrar al-Sham in the town of Harasta and Failaq al-Rahman in the new southern pocket of eastern Ghouta.
More than 1,160 civilians have been killed since Russia-backed regime forces launched a brutal air and ground assault on Eastern Ghouta, the last stronghold of the armed opposition near the capital Damascus.
UNICEF said there were large numbers of civilian casualties reported.
State television on Sunday broadcast from the eastern Ghouta town of Mudeira, which state television and a war monitor said the army had captured to link up with units on the other side of the enclave.
As the Syrian army pushes deeper into Eastern Ghouta under cover of a hammering bombardment, the 400,000 civilians the United Nations says live in the enclave have been crowded in dark basements to cower from the ceaseless bombing.
One rebel commander said the strikes in the south appeared to be a warning to rebels under the Free Syrian Army (FSA) umbrella, who were planning to wage an offensive in coming days to relieve pressure on their comrades in Eastern Ghouta.
According to a UNICEF report released in the German city of Cologne on Monday, the figure includes 86,000 people who have had to undergo amputations.
Across eastern Ghouta heavy aerial strikes continued on the towns of Irbeen, Harasta and other residential areas on Monday, state media said.
People in Eastern Ghouta say they dread a similar fate.
Jaish Al Islam, one of eastern Ghouta's main rebel groups, said on Monday it had reached an agreement with the government's ally Russian Federation to evacuate wounded people, after communicating with Moscow through the United Nations.
Jaysh al-Islam said the evacuations would take place in stages, but it did not say when they would begin.
Meanwhile, at the United Nations, the United States circulated a draft resolution on Monday urging the Security Council to order a 30-day cease-fire in the Syrian capital and eastern Ghouta, expressing "outrage" at the lack of implementation of a resolution adopted February 24 calling for a cease-fire throughout Syria to deliver humanitarian aid and evacuate the wounded and critically ill.
Mattis said Russian Federation, which intervened militarily in Syria to support the Assad government, could be complicit in the civilian casualties. Local doctors have also reported several incidents of bomb attacks followed by the smell of chlorine and choking symptoms.