The UK-based Syrian Observatory for Human Rights says residents of Foah and Kefraya, two government-held villages in the north-west, will be bussed out.
This will be in return for safe passage for people in two rebel-held suburbs of Damascus, Madaya and Zabadani.
About 60,000 people live in the four besieged areas. The deal was brokered by Iran and Qatar.
A truce - which was due to come in effect late on Tuesday - was reportedly agreed to pave the way for the evacuations which are expected to start in a week's time.
A previous attempt to evacuate Foah and Kefraya three months ago was thwarted by rebels, who set buses on fire.
In January, the International Committee of the Red Cross warned that the situation in the besieged areas was "extremely dire".
Activists say civilians have died there because of a lack of food and medicine.
A deal has been reached to allow the evacuation of four besieged areas in Syria, reports say.