Military Chief of Staff Gen. Eduardo Ano said the body of the suspected Indonesian, known by his nom de guerre Mohisen, was recovered by troops along with three slain Filipino followers of militant leader Isnilon Hapilon, who was seriously wounded in the hilly outskirts of Butig town in Lanao del Sur province.
Hapilon was wounded in the arm and was losing blood after air force warplanes, including South Korean-made FA50 fighter jets, unleashed 500-pound (225-kilogram) bombs Wednesday night and Thursday on a militant encampment in an ongoing offensive, Ano said. It was the first time that the FA50s, which were acquired in late 2015 as the military's only fighter jets, were deployed in a combat mission.
Hapilon, who has reportedly been designated to lead an Islamic State group branch in Southeast Asia, was being moved around by his men in a makeshift stretcher but could not escape from Lanao, about 830 kilometers (520 miles) south of Manila, because artillery-backed troops have blocked possible exit points, Ano said.
"We're making it very difficult for them to move around and survive," Ano told The Associated Press by telephone.
The military will ask Indonesian authorities for help in confirming the identity and background of Mohisen, who was not in the list of foreign militants previously monitored as having joined Filipino militant groups in the south.
Hapilon, who is on the U.S. Department of Justice list of most-wanted terrorists worldwide with a reward of up to $5 million for his capture, moved to Butig from his stronghold on southern Basilan island a month ago with about 30 fighters to look for a base for his new militant alliance, Ano said.
IS group commanders apparently wanted Hapilon to set up a base in Lanao, a vast region that offers more security including the presence of other Muslim insurgent groups than his mountain base on Basilan island, so foreign militants can have a springboard to expand their influence in the region, he said.
The ongoing offensive "is significant because it will derail their plan to expand the IS presence to mainland Mindanao," said Defense Secretary Delfin Lorenzana, referring to the southern Philippine region, the scene of decadeslong uprisings by minority Muslims.
President Rodrigo Duterte has repeatedly warned that the emergence of Islamic State-influenced militant groups is fast looming as a major national security threat. While pursuing peace talks with two large Muslim rebel groups in the predominantly Roman Catholic nation, he has ordered the military to destroy smaller but brutal extremist groups like the Abu Sayyaf, which is dreaded for cross-border kidnappings, beheadings and bombings.
Duterte has asked the two Muslim rebel groups in talks with the government not to help extremists under attack by troops, warning that may bring them in a new conflict with Manila.
The elusive Hapilon, an Arabic-speaking Islamic preacher known for his expertise on commando assaults, pledged allegiance to the IS group in 2014.
He then organized an alliance called Dawlatul Islam Wilayatul Mashriq, which is now believed to include at least 10 small militant groups including some Abu Sayyaf factions and the Maute armed group, which he was meeting in Lanao when the military launched the airstrikes using the FA50s and OV-10 bomber planes.
The Maute and the Ansar Al Khilafah Philippines, another group under Hapilon's nascent alliance, have been linked to a Sept. 2 bombing of a night market that killed 15 people and wounded 69 others in southern Davao city, the president's hometown, and a failed bombing at Manila's popular Rizal Park and the U.S. Embassy last year.