A U.S. military mission to train Filipino soldiers fighting Muslim extremists began Thursday as an American diplomat sought to ease fears that U.S. troops will play a more aggressive role.

Robert Fitts, the U.S. charge d'affaires in the Philippines, told 36 American and 100 Filipino troops on a parade square that the exercises will "help eliminate the terrorist parasites who threaten Filipinos as well as the United States."

More than 100 U.S. soldiers are already in the southern city of Zamboanga. Some 500 more, including special forces, are to arrive in coming weeks for the six-month mission to train Filipinos soldiers fighting the Abu Sayyaf, which has been linked to Usama bin Laden's Al Qaeda network and currently holds an American missionary couple.

Filipino officers said the U.S. soldiers will mainly train the local troops on helicopter night flying and in other tactics involving sophisticated equipment.

But some special forces may accompany Filipino soldiers into combat zones and are authorized to shoot in self-defense. Opposition politicians say the mission violates a constitutional clause which restricts foreign combat troops on sovereign soil.

Fitts countered the impression by some politicians and opposition groups that President Bush's State of the Union Address issued a veiled threat to intervene aggressively in the Philippines' war on Muslim extremists.

In his address, Bush said that if countries do not deal with terrorist activities on their soil, "America will."

But Fitts said Thursday that Bush was not referring to the Philippines.

"Nobody can doubt that the Philippine government and the Armed Forces of the Philippines is extremely serious in combatting terrorism," Fitts said.

Fitts also said the exercise was only one of 16 or so planned for this year between U.S. and Philippine soldiers, and one of dozens more in recent years.

About 2,000 U.S. Marines are expected in the Philippines in April for the next joint exercise.

In Manila, a scuffle broke out between police and about 50 leftists in front of the U.S. Embassy in the latest of small, almost daily demonstrations against the exercise. Police made no arrests and the protesters later dispersed without further incident.

Another group of 30 people burned a U.S. flag and a placard with a picture of Bush.

Officials said they overcame last-minute snags on the terms of the current training mission, particularly assurances sought by the Philippines that the Americans would not engage in combat.

The Abu Sayyaf, thought to number as many as 800 fighters mainly on two southern islands, kidnapped Wichita, Kan., missionaries Martin and Gracia Burnham last May 27 along with Corona, Calif., resident Guillermo Sobero and 17 Filipinos from a beach resort.

They beheaded Sobero and several other hostages. The Burnhams remain in captivity, and the others escaped or were released, some reportedly for ransom.

Mrs. Burnham's sister, Mary Jones, left the Philippines on Thursday after making a radio appeal earlier in the week for the Abu Sayyaf to release the hostages.