A military offensive against the Abu Sayyaf group linked to the Islamic State in Iraq and Syria (ISIS) has left 18 soldiers and five militants dead in the worst violence in the Philippines' troubled south this year.
Lieutenant-Colonel Benedicto Manquiquis, spokesman for the army's 1st Infantry Division, said fierce clashes began at 7.55am last Saturday after troops from the division's 4th Special Forces Battalion and 44th Infantry Battalion came across more than 100 Abu Sayyaf fighters in Tipo Tipo town in strife-torn Basilan island, 1,400km south of the capital, Manila.
The firefight lasted for about 10 hours, said Major Filemon Tan, a spokesman for the Western Mindanao Command.
Lieutenant-Colonel Noel Detoyato, head of the military's public information office, said it was a "close- quarter encounter, just 10m".
"It was a terrain that suited the militants but our troops fought valiantly," he said.
Citing sources, the Philippine Daily Inquirer reported that an entire platoon "was wiped out" and four of the soldiers killed were beheaded.
Maj Tan said one of the militants killed was Moroccan Mohammad Khattab, reportedly a bomb-making instructor and extremist preacher. Also reported killed was Ubaida Hapilon, said to be a son of Isnilon Hapilon, Basilan chieftain of the Abu Sayyaf.
Maj Tan said 53 soldiers were wounded. They were airlifted to hospitals in Zamboanga city on the main Mindanao island near Basilan.
At least 20 Abu Sayyaf fighters were injured, including a sub-group leader, Radzmil Jannatul, he said.
The soldiers had been hunting the Abu Sayyaf across Basilan and on nearby Jolo island for two weeks, hoping to free 18 foreigners being held by the group.
Among those being held are Canadians Robert Hall, 50, and John Ridsdel, 68, and Norwegian Kjartan Sekkingstad, 56, whom the Abu Sayyaf had threatened to behead if their demands for ransom were not met by last Friday.
There has been no word yet on the fate of these hostages.
The clashes came a day after retired Italian priest Rolando del Torchio - held hostage by the Abu Sayyaf for six months - was found aboard a ferry docked in Jolo. Some 130 million pesos (S$3.8 million) were reportedly paid for his release.
The Abu Sayyaf is a small but violent band of extremists known for extortions, kidnappings, beheadings and bombings.
It was formed by disgruntled Moro Islamic fighters in 1991, with Al-Qaeda funding.
The latest fighting in the Mindanao island group, home to many of the country's five million Muslims, comes as a peace deal that the government signed with the 12,000-strong Moro Islamic Liberation Front, the Philippines' largest Muslim rebel group, remains stalled in Congress.
Support in Congress for the deal evaporated after a botched security forces raid on a militant stronghold in January last year left 44 elite commandos dead.
"These clashes are largely a reflection of the dangerous power vacuum emanating from the deadlock in peace negotiations and how ISIS sympathisers are intent on establishing a strong foothold in the country," Mr Richard Javad Heydarian, an assistant professor of political science at De La Salle University, told Bloomberg.