GAO, Mali — Black-robed Islamist extremists armed with automatic rifles invaded Gao in wooden boats Sunday to launch a surprise attack on the most populous city in northern Mali, two weeks after French and Malian troops ousted the jihadists.

Gunfire echoed for hours across the city of mud-walled buildings. The combat started at about 2 p.m. in downtown Gao and the fighting was continuing as night fell. Later the sound of gunfire was replaced by the clattering of French military helicopters overhead.