BEIRUT (Reuters) - The Shi’ite populations of two pro-Syrian government towns are to be evacuated in exchange for the evacuation of Sunni rebels and their families from two opposition-held towns in a mediated deal between the warring sides, a pro-government source and the Syrian Observatory for Human Rights said.

FILE PHOTO: A rebel fighter stands with his weapon near evacuees from the Shi'ite Muslim villages of al-Foua and Kefraya as they ride bueses in insurgent-held al-Rashideen, Syria December 20, 2016. REUTERS/Ammar Abdullah

The two Shi’ite 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-government forces.

Its military hand strengthened by Russia and Iran, the Syrian government has struck numerous deals giving besieged rebels safe passage to insurgent-held parts of northern Syria that border Turkey.

The opposition considers it a deliberate policy of demographic change to forcibly displace President Bashar al-Assad’s opponents away from the main cities of western Syria, where he has shored up his rule in the six-year-long conflict.

“This is a demographic change on a sectarian basis,” said Rami Abdulrahman, Observatory director, calling it the biggest population swap agreement of its type. “There is great discontent over the agreement in Madaya,” he said.

The pro-government source said implementation was due to begin on April 4, with 16,000 people leaving al-Foua and Kefraya “in exchange for the evacuation of all the Zabadani militants and the militants of Madaya and their families”.

Sources on the rebel side could not immediately be reached for comment.

As part of the agreement, a ceasefire began on Tuesday at noon, the pro-government source said. The Obsevatory said the areas covered by the agreement were calm on Wednesday.

The Observatory said the evacuation of al-Foua and Kefraya was scheduled to last 60 days. The people evacuated from the rebel-held towns would go to northern Syria, it added.

The agreement would also include a ceasefire covering areas south of Damascus, aid deliveries, and the release of 1,500 prisoners held by the government in connection to the uprising against Assad, the Observatory said.