Syrian rebels on Wednesday seized full control of the southern town of Bosra al-Sham, pushing pro-regime forces out after four days of heavy fighting, a monitor said.

"Rebels and Islamist fighters pushed out National Defence Forces and Popular Defence Committee militias who were defending Shiite districts of the town," said Rami Abdel Rahman, director of the Britain-based Syrian Observatory for Human Rights.

Around 50 percent of the town's residents belong to Syria's Shiite minority, according to the Observatory.

Bosra, which was once the capital of the Roman province of Arabia, was inscribed on UNESCO's World Heritage list in 1980, and placed on the agency's World Heritage in Danger list in 2013.

Abdel Rahman said there had been fighting near the town's archeological zone, but it was unclear whether ruins had been damaged.

Bosra is in Syria's southern Daraa province, where the uprising against President Bashar al-Assad began in March 2011.

The mixed Sunni-Shiite town lies on the main road between Daraa's provincial capital and the city of Sweida, both of which are under regime control.

More than 215,000 people have been killed in Syria since the uprising began, and more than half of the country's population has been displaced.