Philippine officials, dismayed by mounting criticism of U.S. troops joining operations against Muslim guerrillas, said yesterday that American soldiers might not go out in the field at all.

Presidential adviser Eduardo Ermita told reporters the Philippine and U.S. militaries were reviewing how to conduct the joint operations in rebel territory and could decide that the Americans need not accompany local counterparts on patrol or assault missions.

In any case, it would be at least three months before any U.S. soldier would be deployed on the field in the area, other officials said. Although officially non-combatants, sending hundreds of U.S. troops to the Philippines, represents Washington's biggest expansion of the war against terror after Afghanistan.

Despite Manila's protestations, there has been widespread speculation that the U.S. troops would be involved in fighting the Abu Sayyaf, a militant group linked to Osama bin Laden's Al Qaida network and infamous for kidnapping.

President Gloria Macapagal Arroyo reiterated in an interview with CNN that the U.S. troops were in the Philippines to provide training, and not to flush out the Abu Sayyaf.

But many have said it is highly unusual to hold training exercises in areas which could come under attack and that it could just be a cover-up for combat.

Even Vice-President Teofisto Guingona has said he has some reservations over the issue, although he has denied rumours he was resigning from his post as foreign secretary.

"I fully support President Arroyo in the objective of rescuing the remaining hostages of the Abu Sayyaf," Guingona told a local radio station. "But I also expressed some of my concerns." He did not elaborate.

The Abu Sayyaf, based mainly on the rugged and jungle-clad southern island of Basilan, have been holding a U.S. couple and a local nurse hostage there for over seven months.

Meanwhile, a proposal was tabled in the senate to force Arroyo to clarify what the U.S. troops would be doing. It has still to be adopted by the legislature.

Some 650 U.S. troops, including 160 special forces, will participate in exercises with the Philippine military on Basilan and in nearby Zamboanga city at least until June and possibly all year. The exercises were formally launched earlier this week and some U.S. soldiers have already landed on Basilan.

Opposition politicians and left-wing groups here have said the move violates the constitution, which does not allow foreign troops in a combat role in the country.

Former senator Francisco Tatad has said it has made the Philippines "a virtual extension of Afghanistan" and that Arroyo could be impeached for treason.

"People might think the soldiers are there for combat, but they are not there to do combat, they are there to do training... The joint military exercises are intended to be mutual training," Arroyo said in her interview.