"I had to declare martial law in the Mindanao group of islands," Duterte said at a news conference Wednesday. "It is our constitutional duty to enforce the law and provide security."
Duterte's order -- which covers both the city of Marawi and the wider island of Mindanao, of which Marawi is a part -- came after deadly clashes broke out between Filipino government troops and Islamist militants Tuesday.
If the terror threat spreads, Duterte said he's not afraid to go further.