MANILA, Philippines (AP) — The Philippine military suffered its largest single-day combat loss so far this year when 18 soldiers were killed in fierce fighting with Abu Sayyaf extremists that also left five Islamic militants dead, including a Moroccan fighter, the military said Sunday.

At least 53 other soldiers were wounded in Saturday’s daylong clashes with the Abu Sayyaf militants and allied gunmen in the hinterlands bordering the towns of Tipo Tipo and Al-Barka on Basilan island, regional military spokesman Maj. Filemon Tan and other army officials said.

The large combat casualties were reported as the Philippines marked the Day of Valor Saturday to remember Filipino veterans who died in World War II.

Government forces were deployed to kill or capture Abu Sayyaf commander Isnilon Hapilon, who has publicly pledged allegiance to the Islamic State group and has been hunted for years for his alleged involvement in several terrorist attacks, three military officials told The Associated Press Saturday. They spoke on condition of anonymity because they were not allowed to speak publicly about the military assault.

Washington has offered a reward of up to $5 million for information leading to Hapilon’s capture and prosecution.

The Abu Sayyaf militants, many of whom were armed with M203 grenade launchers, apparently managed to reinforce their ranks quickly as the fighting raged and gathered between 100 and 150 fighters, allowing them to inflict heavy casualties on government troops, the officials said.

The fighting lasted for more than nine hours, they said.

Among the slain militants was a Moroccan, who was identified as Mohammad Khattab, and a son of Hapilon, Tan said, adding about 20 other gunmen were wounded.

It’s the largest single-day government combat loss this year in the south, where the military has been battling Muslim separatist rebels and extremists, and Marxist guerrillas.

Last year, 44 police commandos were killed in clashes with various Muslim insurgent groups in southern Mamasapano town while on a covert mission that was fraught with faulty planning and execution but nevertheless killed a top Malaysian terror suspect in Southeast Asia.

Outrage over the police deaths stalled a peace deal with the largest Muslim insurgent group some of whose fighters got entangled in the fighting.

The Abu Sayyaf was founded in 1991 in Basilan, about 880 kilometers (550 miles) south of Manila. With an unwieldy collective of preachers and outlaws, it vowed to wage jihad, or holy war, but lost its key leaders early in combat, sending it on a violent path of extremism and criminality.

The United States and the Philippines have separately blacklisted the Abu Sayyaf as a terrorist organization for carrying out deadly bombings, extortion, kidnappings for ransom, and beheadings of locals and foreigners, including Christian missionaries in the south.

More than a decade of U.S.-backed Philippine offensives have weakened the Abu Sayyaf, but it remains a key security threat.

The brutal group has been blamed for a spike in kidnappings for ransom in recent weeks, including the separate abductions of 10 Indonesian tugboat crewmen near southern Tawi Tawi province and four Malaysian tugboat crewmembers off Malaysia’s Sabah state on Borneo Island.

An Abu Sayyaf faction freed a former Italian missionary on Friday after six months of jungle captivity on Jolo island in Sulu province, also in the south. There has been speculation that a huge ransom was paid to secure the freedom of Rolando del Torchio, who was flown to Manila on Saturday, but the military said it was unaware of any such payment.

The militants are believed to be holding three Filipinos and 18 foreigners, including two Canadians and a Norwegian, mostly in their jungle encampments on Jolo Island.