Syrian rebels have launched a surprise offensive against Syrian President Bashar al-Assad’s regime, a significant escalation in Syria’s civil war, which is raising concerns about a potential wider regional conflict in the Middle East.

BEIRUT—Syria’s President Bashar al-Assad is facing the most serious challenge to his grip on power in nearly a decade after rebels captured most of the city of Aleppo in a swift offensive that crumpled his Russian- and Iranian-backed forces and has the potential to alter the balance of power in the Middle East.