The Britain-based Syrian Observatory for Human Rights announced the deal. It said the evacuation of Al Foua, Kefraya, Zabadani and Madaya might not begin before April 4, but local ceasefires have gone into effect.
Four Syrian towns that have been under a crippling siege for more than two years are to be evacuated under a deal reached late on Tuesday, a monitoring group said.
The Britain-based Syrian Observatory for Human Rights said the deal was brokered by rebel supporter Qatar and regime ally Iran.
The two Shia towns of Al Foua and Kefraya in the northwestern province of Idlib are besieged by rebel fighters, while the opposition-held towns of Zabadani and Madaya near the Lebanese border are besieged by pro-regime forces.
TRT World's Abubakr al Shamahi reports from the city of Gaziantep near the Turkey-Syria border.
Ceasefire for towns in effect
Observatory chief Rami Abdel Rahman said the evacuations are not expected to begin until April 4 but, as a goodwill measure, a ceasefire for the towns came into effect overnight.
"It is calm there now," he added.
The Observatory said the evacuation of Foua and Kafraya was scheduled to last 60 days. The people evacuated from the rebel-held towns would go to northern Syria, it added.
More than 400,000 people have been killed and millions more displaced since the Syrian conflict erupted with protests against Syrian regime leader Bashar al Assad in March 2011.