The United States is sending about 3,000 troops to engage in a major combat offensive in the southern Philippines aimed at wiping out the militant Muslim group Abu Sayyaf, Pentagon officials said yesterday.

The move marks the second time in less than a year that the Bush administration has committed a significant number of U.S. forces to try to root out the extremist group, which has continued to unsettle the Philippines and to target Americans on the islands. It opens another battlefront as U.S. forces already are stretched thin preparing for a possible war in Iraq, securing Afghanistan and pursuing al Qaeda around the world.

Last year, nearly 1,300 U.S. advisers and support personnel participated in what was billed as a six-month training mission to bolster the counterterrorism capabilities of Philippine forces. That effort, which focused on the island of Basilan and concluded as scheduled on July 31, was credited with killing or capturing some Abu Sayyaf members, but it also ended up scattering scores of rebels to other islands.

This time, Pentagon officials are describing the mission not as a training exercise but as a combat operation with no set termination date. Although Philippine forces will have the lead, they will be accompanied in the field by American troops who will remain under U.S. command and be at some risk, defense officials said.

"The intent is for U.S. troops to actively participate," a Pentagon spokesman said. "At this point, we're going into it saying the mission will go on until both sides agree it is finished."

Plans call for U.S. military assessment teams to begin arriving on the island of Jolo in the southern Sulu Archipelago "within days," the spokesman said, with the rest of the U.S. force likely to follow in about a month. The U.S. contingent will consist of about 350 Special Operations troops in the Sulu area and about 400 support personnel in Zamboanga on the island of Mindanao, where the Philippine military maintains a regional headquarters.

In addition, two U.S. amphibious assault ships with 1,300 sailors and 1,000 Marines armed with Cobra attack helicopters and Harrier AV-8B planes will sail from Japan to the waters around Jolo to provide air support, logistical assistance and medical help and to serve as a "quick reaction" backup force. The American forces will be led by Maj. Gen. Joseph Weber, the commander of the 3rd Marine Division, based at Okinawa.

Key members of Congress were notified of the operation yesterday by Pentagon officials. Several Defense Department spokesmen said that they knew of no plans to seek congressional approval for the operation, but added that such a decision rested with President Bush.

The Pentagon has used the Bush administration's global war on terrorism as a rationale for intensifying military ties with the Philippines, contending that the presence of rebel and criminal groups throughout Southeast Asia attracts international terrorists.

Abu Sayyaf has fought for a decade to establish an Islamic state in the southern Philippines, employing tactics that have included kidnappings, extortion and assassinations. The State Department designated the group a terrorist organization in 1997, and U.S. officials have alleged that it is loosely linked to al Qaeda.

But those links have appeared somewhat dated and tenuous. More recently, the justifications offered by U.S. officials for going after Abu Sayyaf have centered on its threat to U.S. interests in the region and on a formal request for help from the government of President Gloria Macapagal Arroyo.

In one of their most publicized attacks, Abu Sayyaf rebels seized American missionaries Martin and Gracia Burnham in 2001 and held them for more than a year. Gracia Burnham was freed by Philippine forces in June but her husband died in the rescue attempt. In October, a U.S. Special Forces soldier was killed along with two Filipinos when a bomb attributed to Abu Sayyaf exploded outside a food stand in Zamboanga near a Philippine military base housing a contingent of U.S. soldiers doing humanitarian work in the area.

Pentagon officials say last year's U.S. military effort, while failing to wipe out the group, did succeed in dislodging it from Basilan and restoring calm to the island. It also appeared to breathe new life into a military alliance with the Philippines that had been battered a decade earlier when U.S. forces were evicted from island bases.

But in wrapping up that operation, Defense Secretary Donald H. Rumsfeld expressed concern about tying U.S. military advisers to a long-term role in the Philippines at a time of competing demands for counterterrorism operations elsewhere in the world, not to mention the possibility of U.S. military action against Iraq. Other U.S. officials also noted then that the sustained involvement of American troops in the Philippines, particularly in combat operations, remains a sensitive political issue in the island nation.

A new security assistance plan worked out in July called for a smaller-scale U.S. military effort, training units on the islands of Luzon in the north, Mindanao in the south and the Visayas in the central Philippines.

But the island of Jolo, located about 70 miles southwest of Basilan, has emerged as another hotbed of Abu Sayyaf activity. Recent clashes between Philippine soldiers and the rebel group have left casualties on both sides. And earlier this month, the Philippine military's chief of staff, Gen. Dionisio Santiago, raised the official estimate of the number of rebels on Jolo from 250 to 500.

Many Filipinos have warned that ensuring security in the impoverished region will ultimately come not through military action but through addressing basic economic needs. Mindful of this, U.S. military authorities supplemented the training mission on Basilan last year with such development projects as digging wells and building roads.

A U.S. CH-53 Sea Stallion helicopter takes off as Marines try to protect themselves and their equipment from the wind yesterday at the Marine base in Ternate in Cavite province, south of Manila.

The first batch of U.S. Special Forces arrive at Zamboanga airport in the Philippines from Okinawa, Japan.