A plan to send 3,000 U.S. troops to the Philippines to track down Muslim separatist guerrillas was left in limbo yesterday after military leaders from both countries failed to find a way to reconcile Philippine law with the prospect of American combat operations in the island nation.

Speaking after talks with his Philippine counterpart, Defense Secretary Donald H. Rumsfeld said both countries remain interested in arranging for expanded U.S. military assistance to Philippine forces combating the Abu Sayyaf rebel group. But he offered no estimate of the size, timing or exact purpose of any U.S. force that might eventually be dispatched.

Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, who appeared beside Rumsfeld at a Pentagon news conference, said the commander of U.S. forces in the Pacific has been asked to prepare other options that would be more in accordance with the Philippine constitution, which prohibits combat activities by foreign troops except in self-defense.

The freezing of the original plan, only a week after a Pentagon spokesman had detailed it to journalists, was an embarrassment for Manila and Washington and a setback for the Bush administration's effort to widen its global war on terrorism. Government sources familiar with weeks of negotiations indicated that Philippine military authorities may have misjudged the extent to which the legal issues could be finessed and led U.S. officials to believe so, too. In any case, both sides clearly had neglected to settle on how to characterize the operation before its disclosure as a done deal last week.

Philippine Defense Secretary Angelo Reyes, at a separate news conference before his noontime meeting with Rumsfeld, described the basic problem as essentially "one of definitions and semantics." He said that while his government saw the proposed involvement of U.S. forces as "a training exercise," the Pentagon was compelled to call it a combat operation, since U.S. military exercises are not usually held in hostile, guerrilla-infested areas like the Sulu Archipelago in the southern Philippines, which is being targeted.

"That we are groping for the correct solution to the legal problem is correct," Reyes said.

But simply figuring out how to label the role of American troops may not be enough. The notion of U.S. troops engaging directly in field operations has kindled controversy in the Philippines, a former American colony that voted a decade ago to shut down two major U.S. bases and assert its independence. Any attempt to stretch legal limits is sure to invite court challenges.

At the same time, Reyes reaffirmed his government's strong interest in obtaining more U.S. counterterrorism help. And yesterday, in the Philippines, President Gloria Macapagal Arroyo underscored the urgency of the issue, announcing that she had given the military 90 days to vanquish Abu Sayyaf. Accompanying her during a visit to a military camp outside Manila, Gen. Dionisio Santiago, chief of staff of the armed forces, warned that commanders who fail to perform will be replaced.

Asked about the deadline, Reyes said that it "would be far easier" to meet "if we are assisted and supported by American forces."

Formed a decade ago with the aim of establishing an Islamic state in the southern Philippines, Abu Sayyaf has evolved into essentially a kidnapping gang of several hundred members. U.S. military officials have been interested in crushing the group, both because of evidence of links to al Qaeda and other terrorist organizations and because the group's violent tactics have undermined the economic and political stability of a key Asian ally. Pentagon strategists worry that the lack of law and order in the southern Philippines provides a potential haven for other terrorist activity.

With the assistance of U.S. Special Forces last year, Philippine troops drove Abu Sayyaf members from the island of Basilan. But in that six-month operation, the Americans were limited largely to training Philippine troops in barracks at the battalion level.

Under the plan unveiled last week, about 350 U.S. Special Forces members were to join more than 4,000 Philippine marines and army soldiers on potentially dangerous combat patrols in the Sulu region, mostly on the island of Jolo -- about 70 miles southwest of Basilan. Another 400 support troops were to be based in Zamboanga on the island of Mindanao. In addition, 1,000 Marines on two ships manned by 1,300 sailors were to be positioned in the waters off Jolo to provide air cover and emergency backup.

A 1999 Visiting Forces Agreement allows foreign forces to train Philippine troops. But a ruling last year by the Philippine Supreme Court, issued in response to a legal challenge to last year's U.S. training effort, specified that foreign troops could not take part in offensive combat operations and could fire weapons only in self-defense.

A key issue to be worked out, Reyes said, is whether U.S. troops would be prohibited from opening fire except when fired on, or whether the Americans would be allowed to shoot while simply under threat of potential attack.

Rumsfeld expressed confidence that a new counterterrorism operation would be worked out and that "it very likely will have an intelligence component, a command-and-control component, a training component, some exercises. And whatever it ends up being, it will clearly be consistent with" the Philippine constitution.

Pressed on whether there would still be joint combat operations, Rumsfeld said: "The fact is that the way you phrased it would be perfectly comfortable from our standpoint. From their standpoint, it would be inconsistent with their constitution. Therefore, what we have to do is find an approach where we can provide the maximum benefit to them and do it in a way that is not inconsistent with their circumstance."

Reyes, in a phone interview late yesterday afternoon, said his meeting with Rumsfeld had "clarified a lot of matters." Asked how the confusion had arisen in the first place, he said details of the operation had been released in Washington before they were finalized.

Rumsfeld offered a similar explanation, although the first official word of the operation emerged early last week not in Washington but Manila, where Arroyo's spokesman, Ignacio Bunye, announced that U.S. forces would be participating in a joint "exercise" on Jolo. Concerned that the Philippine statement did not properly portray the operation, senior Pentagon officials authorized a spokesman to brief reporters on the plan and to make clear that U.S. troops would "actively participate" in combat against Abu Sayyaf.

What followed was a weeklong exchange of conflicting U.S. and Philippine descriptions of the American role -- until yesterday, when Rumsfeld indicated that the operation was being reconsidered.

Correspondent Ellen Nakashima in Manila contributed to this report.