President Gloria Macapagal-Arroyo said yesterday that the plan to hold joint military exercises with the United States is "not unconstitutional", adding the U.S. troops will not participate in actual combat against the Abu Sayyaf bandits who are still holding an American missionary couple and a Filipino nurse in jungle hideouts in Basilan.

The deployment was within the bounds of the 1987 constitution and American soldiers will only be acting as advisers, Arroyo told local radio.

Arroyo said the war games were meant to be "more responsive and attuned to the times", referring to the U.S.-led global war against terrorism, following the terror attacks in the U.S. last September 11.

"Our enemy is the Abu Sayyaf, who are international terrorists. They are not only an internal threat, they are an international threat," Arroyo said.

Pro-administration Senator Franklin Drilon said however that he will ask the vice president and Foreign Affairs Secretary Teofisto Guingona to explain why the six-month joint Philippine-U.S. military exercises in Mindanao, southern Philippines, were being held at all.

"I'm going to file a resolution in the senate and request the vice president to explain to the public what the six month war games is all about," said Drilon.

"It is a little out of the ordinary," said Drilon, adding the senate is exercising its function to find out if the Visiting Forces Agreement with the U.S. is being implemented properly.

Opposition and former defence secretary Juan Ponce Enrile said that American involvement may escalate into an "intrusion" similar to that in Vietnam in the early 1970s.

The administration is simply using the term "American advisers" to hide the real objective of the presence of American troops in the country, said Enrile.

"We have the potential of a Vietnam conflict. If reports are true, this is the first time the Philippine government has authorised foreign troops to fire at Filipino citizens," Enrile noted.

The presence of a large number of American troops in the Philippines may be tantamount to "stationing troops" which is a violation of the VFA and the Constitution of the Philippines, said Enrile.

The Philippine Constitution Association also asked Arroyo to explain why she has authorised American troops to directly participate in the rescue operations for the hostages.

The U.S. advisers could merely worsen the peace and order situation in Mindanao, saying the Muslims would resent foreign intrusion in their homeland, said Sanlakas, a leftist political party.

The incompetence of the Armed Forces of the Philippines (AFP) has become a convenient excuse for the Americans to violate the Philippine Constitution, said leftist Bayan, adding the Abu Sayyaf was created by Philippine and the U.S. officials in the early 1992 in a bid to undermine the secessionist struggle of separatist groups in the southern Philippines.

"This is not a simple military exercise anymore. It's more of a tolerated invasion," said Sanlakas president Wilson Fortaleza.

After the predicted annihilation of the Abu Sayyaf Group in Basilan and Jolo, the Philippine and American troops will then train their sights on the communist NPA rebels and the Pentagon gang operating in Central and Western Mindanao, said Defence Secretary Angelo Reyes.

The rest of the U.S. troops will join their Philippine counterparts at the training facility in Zamboanga City and in Basilan island.