Muslim extremists abducted a Catholic priest and more than a dozen churchgoers while laying siege to a southern Philippine city overnight, burning buildings, ambushing soldiers and hoisting ISIS flags, officials said on Wednesday. President Rodrigo Duterte has declared martial law in the southern third of the nation and warned he would enforce it harshly.

The violence erupted on Tuesday night after the army raided the hideout of Isnilon Hapilon, an Abu Sayyaf commander who is on Washington's list of most-wanted terrorists with a reward of up to $5 million for his capture. The militants called for reinforcements from an allied group, the Maute, and some 50 gunmen managed to enter the city of Marawi.