Philippine troops seized the main base of the Abu Sayyaf group on the southern island of Jolo, but eight soldiers were killed on Monday when guerrillas ambushed a convoy returning from the area, the military said, Reuters reported.
At least five rebels were killed in the fighting, Lieutenant Colonel Romeo Brawner told reporters.
"Our troops were returning to the main marine base after taking part in a flag-raising ceremony to symbolize the capture of a rebel camp when they were ambushed not far from the area," he said.
"We lost eight soldiers but we got five of them in the gunbattle."
On Sunday, troops backed by air strikes seized the base near Indanan town on Jolo, killing 19 rebels in a complex laced with bunkers and trenches, another military officer said.
Hundreds of soldiers, supported by OV-10 Bronco planes, fought for more than six hours to dislodge about 200 Abu Sayyaf rebels from fortified hills near Indanan, Lieutenant-General Ben Dolorfino told reporters.
"We had to call in air strikes because the soldiers were having difficulty in scaling the steep terrain. The air force dropped 16 250-lb bombs in two sorties," Dolorfino said.
"Our troops were tracking down a senior Abu Sayyaf leader when they stumbled into the main rebel base on Jolo," he said, adding five soldiers were wounded in the initial assault.
Jolo is the stronghold of the Abu Sayyaf rebels, one of the smallest but deadliest Islamic militant groups in the largely Roman Catholic Philippines.
Dolorfino said the soldiers were able to seize the rebel base after the second round of air strikes.
"Two bodies were recovered, but we were told by our intelligence people that 17 others had been killed in the air strikes," he added.
Dolorfino said the captured rebel base could accommodate about 500 people in fortified bunkers and a network of trenches.
Dolorfino said the military could have pre-empted a key meeting of the Abu Sayyaf because three of its top leaders -- Umbra Jumdail, Albader Parad and Isnilon Hapilon -- were in the area when the fighting erupted.
Two known members of the Southeast Asian militant network Jemaah Islamiah, including the Singaporean Mauwiyah, were said to be in the same rebel base.