Out of the almost 400,000 people besieged in eastern Ghouta for years by Syrian government forces, 130,000 had fled in the last three weeks, Egeland said.
Government forces along with its Russian ally launched a relentless ground and aerial offensive in February 18 to retake the Damascus suburb it had lost in mid-2013.
At least 50,000 people have been evacuated from eastern Ghouta since the evacuation process began on March 22.
Ahmad Abazeid, an independent Syrian analyst, said rebels in Ghouta's armed insurgency had been a rare case of more independent decision-making. Russian Federation and Iran have provided crucial support to President Bashar Assad's forces, while Turkey has backed the rebels seeking to overthrow him.
Syria has been locked in a devastating conflict since early 2011 when the regime cracked down on demonstrators with unexpected ferocity.
In Syria, the mass evacuation of civilians is threatening to leave aid workers "overrun" and "acutely underfunded", as details emerge of the unparalleled destruction in urban areas like the former ISIL-held city of Raqqa, UN senior adviser Jan Egeland has said.
There was no comment from the Army of Islam.
The Britain-based war monitor said the insurgents had kidnapped them from the nearby town of Adra about five years ago. Security forces responded by putting the town and other suburbs around Damascus under siege, bombing hospitals and residential areas, and blocking the entry of food and medical relief.
Eastern Ghouta, a 105-square-km agricultural region consisting of several towns and farmlands, used to be occupied by several rebel groups, who posed the last threat to the capital by firing shells into Damascus' government-controlled neighborhoods.
The rebels are also coming under intense pressure from the estimated 70,000 civilians trapped inside Douma who are desperate for the fighting to end, our correspondent adds.
Fighters from the last rebel-held town in Eastern Ghouta leave for Idlib province after weeks of uncertainty.
Ahmad Ramadan says Turkey, which backs various Syrian rebel groups, and shares control over a part of opposition territory in north Syria, is also party to the talks.
On Sunday, the Syrian government-linked Central Military Media outlet said that once the evacuations are completed, a local council for Douma will be formed with the approval of the central government. The UN and rights groups have criticised the regime and its Russian ally for the heavy bombardment of civilians areas and for not implementing a February 30-day ceasefire in Eastern Ghouta.