Buses evacuated thousands of people from two rebel-besieged Shia villages in northwest Syria on Friday and rebels began to leave two towns near Damascus with their families, under a deal between the government and insurgents.

Similar agreements have been reached in recent months, with Syrian rebels leaving areas long-besieged by President Bashar al-Assad's forces, sometimes in exchange for Shia Muslim residents moving from the villages surrounded by the mostly Sunni insurgents.

After six years of fighting a civil war, Damascus has gained the upper hand against rebels in the west of the country, and been able to negotiate the deals from a position of strength thanks to Russia's intervention since 2015 and increased support from Shia allies Iran and Lebanese Hezbollah.

The opposition says the deals amount to forced demographic change and deliberate displacement of Assad's enemies away from the main cities of western Syria. The government says the deals allow them to take back control and to restore services in the wrecked towns.

Early on Friday, residents of the mostly Shia villages of al-Foua and Kefraya, besieged by rebel forces in the insurgents' northwestern Idlib province stronghold, left on dozens of buses, the British-based Syrian Observatory for Human Rights monitoring group said.

Similar agreements have been reached in recent months, with Syrian rebels leaving areas long-besieged by President Bashar al-Assad's forces, sometimes in exchange for Shia Muslim residents moving from the villages surrounded by the mostly Sunni insurgents. (Ammar Abdullah/Reuters)

The buses arrived several hours later on the outskirts of government-held Aleppo city in northern Syria, the Observatory said.

Meanwhile, buses carrying rebel fighters and their families left the government-besieged town of Madaya near Damascus and close to the Lebanese border, the Observatory and a pro-Damascus military media unit reported.

The evacuation of nearby Zabadani, another town surrounded by government forces and their allies and included in the deal, appeared to have been delayed. No buses had yet left the town, but that evacuation was expected to begin later on Friday.

The convoys from Madaya and Zabadani are to head for Idlib.

A member of one of the Shia parties said 60 buses were moving through the town of al-Foua. A similar number of buses were leaving Madaya, the Observatory said. State television reported that engineering teams and Syrian forces would soon enter the town.

Damascus holds upper hand

About 5,000 people were being transported from the Shia villages, and more than 2,000 from Madaya. The convoys include hundreds of fighters from each side, the Observatory said.

Syria's population is mostly Sunni Muslim. Assad is from the Alawite religious minority, often considered an offshoot of Shia Islam.

Assad's forces and their allies have fought rebels for six years in the conflict that grew from a popular uprising in 2011.

Russia's intervention around 18 months ago has helped Assad gain the upper hand militarily, despite diplomatic pressure and support for the rebels by Western and Gulf Arab states.

Rebels and Islamist factions have fought back and achieved recent advances in some areas.

The United States escalated its involvement in the conflict last week, striking an air base in Washington's first deliberate direct attack on Syrian government forces.

Assad achieved his most significant victory to date in December, driving rebels out of long-divided Aleppo.