The Syrian army and allied forces have recaptured 70 percent of the territory that was under insurgent control in the enclave and after weeks of bombardment residents are fleeing by the thousands.
The relentless Syrian army assault backed by Russian air power that began last month has killed hundreds of people as relentless airstrikes pounded residential areas where thousands had sheltered in basements across the densely populated enclave, according to rescuers and a monitor.
The evacuation deals come after years of siege and bombardment that have been a major strategy by the Syrian army to force rebels to surrender and help Syrian President Bashar al-Assad recover all of Aleppo, Homs and other areas.
"The deal has been finalised and it could come into effect soon after a ceasefire is announced as early as Wednesday," said one official familiar with the talks that have been going on for several days.
It would begin with an evacuation of injured civilians, he added, saying the remaining civilians in the town were "facing untold suffering."
A local official in the opposition-run Harasta council was quoted by opposition news outlets that a deal had been reached but did not say when it was expected to be implemented.
Last year, rebels launched a major offensive on army barracks on the edge of Harasta that led to retaliatory attacks. The battles were among the fiercest in Eastern Ghouta in recent years.
Assad has vowed to end what he portrays as a terrorist threat in close proximity to his seat of power.
On Tuesday, a rocket fell on a popular residential area across the rebel-held enclave killing at least 40 people, according to medics in government controlled areas.
The Syrian authorities accused rebels of firing the rockets in revenge attacks which rebels deny.
Over 100 civilians were killed in the last two days of air strikes in Eastern Ghouta with most of the raids on Douma city, the largest population centre with more than 150,000 people still living there.
Rebels and residents say napalm and incendiary weapons were dropped on several civilian areas to force rebels to surrender.
The predicament of hundreds of rebels trapped in Harasta, one of the main towns in Eastern Ghouta, worsened after the army succeeded earlier this month in splintering Ghouta into three besieged zones, cutting off Harasta from other areas.
Last Sunday, the Syrian army had given the rebels of Harasta area an ultimatum to withdraw, state media said.
Residents and rescuers say the Russian airforce stepped up its bombing of Harasta town as talks were going on to broker the deal.
The Harasta deal will pile pressure on the two main rebel groups - Failaq al-Rahman in the southern pocket and Jaish al-Islam in the northern enclave - to also reach understandings.
They have however publicly said they reject Russia's offer to leave the enclave.
The most likely option was however the transfer of Failaq al-Rahman and Jaish al-Islam fighters to opposition-held areas in northern and southern Syria respectively, a rebel official familiar with the situation on the ground said.
The defeat in Eastern Ghouta would mark the worst setback for the anti-Assad rebellion since the opposition was driven from eastern Aleppo in late 2016 after a similar campaign of siege, bombing, ground assaults and the promise of safe passage out.
(Reporting by Suleiman Al-Khalidi; Editing by Lisa Shumaker and Nick Macfie) ((email@example.com; +962 79 5521407; Reuters Messaging: firstname.lastname@example.org))