The Philippine military said Sunday that 18 of its soldiers were killed and more than 50 others were wounded in a 10-hour firefight with Abu Sayyaf extremists in the southwestern island province of Basilan.
A military spokesman said at least five militants died and 20 others were wounded in Saturday's fighting. He said the dead included a Moroccan national.
The Philippine Daily Inquirer reported that an entire military platoon "was wiped out" and that four of the soldiers were beheaded. The fighting was described as the worst single day of government combat casualties in the region this year.
Authorities said the military force had been hunting the militants in Basilan and nearby Joso island for the past two weeks, in a push to free as many as 18 captive foreigners, including several Canadians and a European kidnapped from a local beach resort last year.
Abu Sayyaf later released videos in which some of the captives pleaded for their governments to secure their freedom. Militants had also threatened to behead their prisoners if their ransom demands were not met by Friday.
Military authorities quoted by The Associated Press said many of the militants engaged Saturday were armed with grenade launchers, and that they were able to reinforce their ranks quickly as the fighting raged near the Basilan towns of Tipo Tipo and Al-Barka.
Abu Sayyaf — a splinter grouping of the now-disbanded Moro Islamic Front — was founded in 1991 with funding from al-Qaida. It has been designated as a terrorist group by the United Nations, Australia, Canada, Indonesia, Malaysia, the Philippines, United Arab Emirates, Britain and the United States.
Its current leader, Isnilon Totoni Hapilon, swore allegiance to Islamic State leader Abu Bakr al-Baghdadi in 2014.