U.S. commanders are watching intently to see how the more than 5,000 Sunni tribal fighters perform in the Iraqi offensive to retake Fallujah, which has been under Islamic State control for two years.

So far the Iraqi assault, which includes elite counterterrorism troops, police and some Shiite militias, has met only light to moderate resistance as it works through mines, booby traps and physical obstacles on the outskirts of the city.