The agreement was announced on Sunday morning by Syria's government and its ally Russia, just hours after toxic gases were allegedly released on Douma.
First responders there said more than 40 people died Saturday after suffering symptoms consistent with chemical exposure, including wheezing, discoloured skin and foaming at the mouth.
Analysts have said Syrian President Bashar al-Assad may have used toxic gas to terrify Douma's residents and push rebels to withdraw without a fight. Jaish al-Islam had long insisted that it would refuse to agree to an evacuation deal for Douma.
But since the alleged attack, several thousand rebels and civilians have left Douma for opposition-held territory in the north.
They included Jaish al-Islam's chief Issam Buwaydani, according to the Syrian Observatory for Human Rights, a Britain-based monitoring group. "Not all the leadership has left yet. Departures are ongoing," Dalwan told AFP.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)