BAGHDAD —Militants from the Islamic State group seized Iraq’s largest hydroelectric dam on Thursday, giving them control of enormous power and water resources and leverage over the Tigris River that runs through the heart of Baghdad. The fighting has trapped tens of thousands of members of religious minorities on a mountaintop.

The dam seizure was the latest in a string of victories by the Sunni radical group as it expands its hold in northern Iraq, driving back Kurdish forces, sending minority communities fleeing and unleashing bombings that have killed more than 90 people.