Hundreds of US troops were confined to their base in the northern Philippines Friday after one of their planes was hit by small-arms fire, allegedly by communist rebels opposed to US anti-terror actions here.

The MC-130 special forces plane was hit by two bullets on Thursday while on a low-flight training mission over a mountainous area on the main island of Luzon, the two governments said.

None of the crew were wounded and the plane returned safely to Clark Air Base, the staging ground of one of two simultaneous joint military exercises in the country.

Philippine officials acknowledge that communist New People's Army (NPA) guerrillas operate in the area as well as in the Pinatubo volcano near Clark, where an American mountain climber was killed in a separate gun attack on Wednesday.

"We are investigating the incident," Filipino Air Force spokesman Major Art Orticio told AFP referring to the attack on the plane. At this point "we can't say who were responsible for this."

A Filipino military source said on condition of anonymity earlier that the ground fire came from Abra. But a Filipino government commission later said the M-14 bullets were fired from the adjacent province of Kalinga.

"It was in Abra and you know there are NPA over there," the source told AFP.

The plane and its crew were taking part in an exercise called Balance Piston that is focused on special operations and counter-terrorism.

The airmen are based at Clark, the former headquarters of the 13th US Air Force that Washington returned to the Manila government in 1992.

The 400-odd US troops have been "ordered to remain inside the base as a result of the shooting incidents," said Jaime Yambao, executive director of the Filipino government commission that oversees the manoeuvres.