Two weeks after a critical victory at Tikrit, where a combination of U.S. airstrikes, government troops and Shiite Muslim militias overwhelmed an Islamic State force that had held the city for nearly a year, the Iraqi government faced a new challenge Wednesday, a sign of how much remains to be done to defeat the militants.

In a surprise assault, Islamic State fighters captured three villages outside Ramadi, the capital of Anbar province 100 miles south of Tikrit, and pushed to within 500 yards of a key government center in the northeastern section of the city, one of the few population centers in Anbar still under government control.

Government forces were responding with heavy bombardment from military aircraft, but the outcome of the battle was uncertain as night fell. Thousands of residents and troops were reported fleeing the city.[…]