MANILA, Philippines — Abu Sayyaf extremists killed 15 Philippine army soldiers in fierce fighting Monday in the country’s restive south, dealing the government its largest single-day combat loss under President Rodrigo Duterte, who ordered the militants to be crushed for their brutality.

Military officials said five other soldiers were wounded in the nearly two-hour gunbattle that also killed at least two militants in Sulu province’s mountainous Patikul town, where the Abu Sayyaf have detained many of their kidnap victims.

The estimated 70 militants were led by Abu Sayyaf commander Radulan Sahiron, a one-armed fighter long wanted by Philippine and U.S. authorities for his role in bombings, kidnappings and other acts, according to regional military commander Maj. Filemon Tan and police officials.

Duterte ordered troops to hunt down and destroy the militants in their jungle bases last week after the extremists beheaded a kidnapped villager whose family was too poor to pay a ransom. The tough-talking president has pursued peace talks with two larger Muslim rebel groups, including the Moro National Liberation Front whose fighters have been suspected of providing sanctuary and combat support to the Abu Sayyaf in Sulu in the past.

The military’s battle setback came after troops killed at least 21 Abu Sayyaf gunmen, including an influential commander, in the jungles of Patikul on Friday and Saturday in assaults that followed the beheading.

The United States and the Philippines have blacklisted the Abu Sayyaf, which is estimated to have more than 400 armed members, for deadly bombings, ransom kidnappings and beheadings. Although the militants have been weakened by years of U.S. military-supported Philippine offensives, they remain a national security concern and have been implicated in the recent kidnappings of Indonesian and Malaysian crewmen of tugboats plying the area around the sea borders of the three countries.