Yasser al-Youssef from the political office of the key Nurredin al-Zinki group told AFP the deal with President Bashar al-Assad's regime was being "sponsored" by Russia and Turkey.
"An agreement has been reached," Youssef said. "The first stage will be the evacuation of civilians and wounded, within hours, and afterwards fighters will leave with their light weapons."
Those leaving will be allowed to travel to other rebel-held territory in the west of Aleppo province or neighbouring Idlib province, he said.
There was no immediate confirmation from the regime, Ankara or Moscow.
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The UN and aid agencies have been pleading for a ceasefire to allow for the evacuation of tens of thousands of civilians trapped in the last pocket of rebel territory in Aleppo.
After weeks of heavy fighting, forces loyal to Assad were in the last hours of a push to take full control of the city, dealing the biggest blow to Syria's rebellion in more than five years of civil war.
But as the long battle reached its final stages, the UN said it had received reports of at least 82 civilians, including 11 women and 13 children, being executed by pro-government forces.
Speaking to reporters in Geneva, UN rights office spokesman Rupert Colville said it had received credible reports of the civilian executions in recent days.
Pro-government fighters had in some cases entered homes and killed those inside, and in others "caught and killed on the spot" fleeing civilians, he said.
The UN was "filled with the deepest foreboding for those who remain in this last hellish corner of opposition-held eastern Aleppo", Colville said.