At least 53 other soldiers were wounded in yesterday'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 Valour yesterday to remember Filipino veterans who died in World War II.
Washington has offered a reward of up to USD 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.
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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.
The Abu Sayyaf was founded in 1991 in Basilan, about 880 kilometres 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 US and the Philippines have separately blacklisted the Abu Sayyaf as a terrorist organisation for carrying out deadly bombings, extortion, kidnappings for ransom, and beheadings of locals and foreigners, including Christian missionaries in the south.