At least five members of the Muslim extremist Abu Sayyaf group were killed early Friday in a gun battle on Basilan island, where the guerrillas are holding two Americans and a Filipino hostage, a military commander said.

The firefight came a day after the Philippines and the United States formally opened a training exercise aimed at wiping out the Abu Sayyaf, which has been linked to the Al Qaeda terror network.

Soldiers from the Philippine Marine Battalion Landing Team 5 encountered about 20 guerrillas and engaged them in a 15-minute gunfight in a village of Maluso town on Basilan island near Zamboanga, said Southern Command chief Lt. Gen. Roy Cimatu.

"I would say that this group is the blocking force of the main Abu Sayyaf group that is holding the hostages," Cimatu said. "If it is ... probably we will have some positive developments in the near future."

Troops recovered the body of one guerrilla, five assault rifles, personal belongings and ammunition in the village of Las Poler. The rest of the guerrillas escaped with their other casualties and were being pursued by the marines, who suffered no injuries, Cimatu said.

"We have destroyed their perimeter defense ... [while] degrading their capability in terms of strength and firepower," he said.

The guerrillas have eluded about 7,000 soldiers deployed to Basilan since the Abu Sayyaf went on a kidnapping spree last May. The guerrillas are still holding Christian missionaries Martin and Gracia Burnham of Wichita, Kan., and nurse Deborah Yap of Basilan.

On Thursday, U.S. troops kicked off a controversial six-month training exercise for Filipino soldiers that U.S. charge d'affaires Robert Fitts said will "help eliminate the terrorist parasites" threatening both countries.

The exercise will involve 660 U.S. soldiers, including 160 from the Special Forces who will be deployed in 12-man teams among the three Philippine marine and five Philippine army battalions on Basilan.

Some of the Special Forces troops may accompany Filipino soldiers into combat zones and are authorized to shoot in self-defense. Opposition politicians say the mission violates a Philippine constitutional clause that restricts foreign combat troops on sovereign soil.

In Manila, two lawyers from the southern Mindanao region asked the Supreme Court to stop the exercise.

In their petition, Arthur Lim, former national president of the Integrated Bar of the Philippines, and Paulino Ersando said no provision in a bilateral Visiting Forces Agreement allows "U.S. troops to be deployed even only as a trainer or an adviser or as a chaperon for Filipino troops in a war zone area like the one in Basilan."

More than 100 U.S. soldiers are already in the southern port city of Zamboanga. Some 500 more, including the Special Forces, are to arrive in coming weeks.

Adm. Dennis Blair, commander in chief of the U.S. Pacific Command, said in neighboring Malaysia on Thursday that he was told by the Philippine government that the objective was to destroy the Abu Sayyaf and not to drive them away. The guerrillas snatched a batch of tourists from a Malaysian resort in 2000 before releasing them for huge ransoms.