The militants have built new court systems to enforce their harsh interpretation of sharia law, which punishes thieves by amputating their hands and has sentenced numerous Christians and other religious minorities to death because of their beliefs, the officials added.

At the same time, the Islamic State has generally allowed the local bureaucrats in charge of hospitals, law enforcement, trash pickup, and other municipal services to stay in their jobs, according to intelligence officials. In some areas, sitting mayors and other top local officeholders are keeping their posts.

Taken together, the moves highlight the fact that the Islamic State, already the best-armed and best-funded terror group in the world, is quickly adapting to the challenges of ruling and governing. That, in turn, dramatically reduces the chances that the extremists will face homegrown opposition in what amounts to the world’s newest territory.