Lieutenant Colonel Ariel Caculitan, a marine spokesman, told Reuters that the base was seized after a protracted firefight Saturday morning that left 15 troops and at least 20 militants dead. The facility was laced with underground bunkers, tunnels and well-developed trenches, Caculitan said.

The military operation on Basilan was prompted by an encounter there July 10, when insurgents killed 14 members of the Philippine Marines who marched into a village known to be an Abu Sayyaf stronghold. Ten marines were later found beheaded.

In the past month, the government has increased the number of troops on Basilan and the adjacent island province of Sulu to more than 12,000, the biggest such deployment since 2001.

"The firefight is ongoing," Lieutenant Colonel Bartolome Bacarro said in a news briefing Saturday. "Our troops are now concentrating in the area. We will press on with the fight."

In 2002, the United States sent hundreds of troops to Basilan to help Philippine troops destroy Abu Sayyaf, a terrorist group with alleged links to Jemaah Islamiyah, the Southeast Asian terrorist network responsible for several attacks in the region since 2001, including the 2002 nightclub bombings in Bali.

Since then, both Washington and Manila have routinely claimed success on Basilan, saying that top Abu Sayyaf leaders had been killed and the group's military infrastructure largely neutralized on the island. They also claimed to have reduced the risk of a resurgence through a campaign to win over the local population with nonmilitary development assistance.

In Basilan, U.S. and Filipino forces worked together "to eradicate Abu Sayyaf group havens on the island through a combination of civil-military operations and improved counterterrorism coordination," Henry Crumpton, Washington's counterterrorism chief, said during a news conference in Manila in October 2005. "This model offers a highly successful example of what we can do together."

According to the Philippine military, the number of Abu Sayyaf fighters on Basilan has steadily dwindled to about 200 last year from a high of more than 1,000 in 2000. Both governments credited the nonmilitary assistance program provided by Washington, which built bridges, schools and clinics throughout the island, with winning the hearts and minds of Basilan people.

The recent resurgence of Abu Sayyaf activity there, however, has raised questions about the reality of these gains. Some analysts specializing in the region said the Basilan campaign had been prematurely curtailed in 2003 when the bulk of military resources were shifted to Sulu, where Abu Sayyaf militants are also active.

Zachary Abuza, a specialist in Southeast Asian terrorism at Simmons College in Massachusetts who is writing a book on Islamic separatism in the Philippines, said the Sulu campaign had much more support from Washington, which maintains an undetermined number of troops on the island.

"I think the U.S. and armed forces of the Philippines have thrown everything they have into Jolo," he said in an interview, referring to the main island in Sulu Province. "They are thin elsewhere."

When in January the Philippines announced that it was pulling troops from other provinces and sending them to Basilan, Abuza said, "that was a clear indication that things were not going as well there as everyone was saying."

"I don't think that what the armed forces of the Philippines and the U.S. did in Basilan was a failure," Abuza said, but they departed hastily.

Earlier this month, the Philippine military suffered its highest casualties in recent years during separate battles on Jolo Island with fighters suspected of belonging to Abu Sayyaf. Twenty-seven soldiers were killed in those clashes.

Western officials, speaking before the fighting Saturday, warned against gauging success in the Basilan theater by the beheading incident alone.

"The government's system in Basilan is now functioning," said a senior Western military official, who asked not to be identified because he is not authorized to provide public assessments of the U.S. military effort.

A Western aid official, who also asked not be named, said of Basilan: "Most people there think that they're much better off today. Their lives are improving. They have health care."

But the continued presence of Abu Sayyaf on Basilan could prove embarrassing both for Manila and Washington, according to Abhoud Syed Lingga, executive director of the Institute of Bangsamoro Studies, which researches issues concerning Filipino Muslims.

"Exactly what benchmark did the government use in determining success in its operations against the Abu Sayyaf in Basilan?" Lingga asked in an e-mail exchange. "If the measure is the Abu Sayyaf's absence, they can come back after the military operations were over."