MANILA, Philippines -- A nail-packed bomb killed an American Green Beret and two Filipinos on Wednesday outside a restaurant near a base in the troubled southern Philippines, where the U.S. military helped in the fight against al-Qaida-linked rebels this year.

The blast, from a bomb hidden on a motorcycle, wounded 25 people outside the restaurant, which is frequented by U.S. and Filipino soldiers, in the city of Zamboanga, officials said.

Television footage showed a pool of blood and unconscious victims -- some with their shirts bloodied -- being loaded into ambulances.

No one claimed responsibility for the blast. Suspicion fell on Muslim extremists like the al-Qaida-linked Abu Sayyaf group and communist rebels who had threatened earlier in the day to attack police and military installations.

Security had already been tightened in advance of an Oct. 12 Christian festival in the middle of the southern islands that make up the archipelago's Muslim heartland. Amid worries over further attacks, more troops were being sent in, and checkpoints were set up on major roads and outside the city's power plant.

The 9 p.m. blast in Zamboanga ripped the roof off a small wooden house and damaged six shops across the street from the Camp Enrile army base, where some U.S. troops have been staying.

One of the Filipinos killed was the driver of the motorcycle, who "is suspected to have been the one who brought the bomb," said Defense Secretary Angelo Reyes.

Army Col. Alexander Yapching, head of Task Force Zamboanga that is in charge of securing the city from terrorist attacks, said a U.S. Army master sergeant died en route to a hospital and another American was injured, along with five Filipino troops.

The wounded American's injuries did not appear to be life-threatening, said a high-ranking Philippine military source. The Pentagon confirmed that one American soldier was killed and another wounded.

The dead and injured Americans were Green Beret soldiers on duty at the time of the blast, said Lt. Cmdr. Barbara Burfeind, a Pentagon spokeswoman. The Green Berets are providing anti-terrorism training to the Philippine armed forces, she said. The identities of the Americans were not immediately released.

Bomb experts collected debris at the site, including a large number of nails from the bomb. Only the charred handlebars and frame remained of the motorcycle. Two Americans in civilian clothes stood nearby talking on cell phones. Philippine soldiers walked sniffer dogs in the area.

"We should be careful," Yapching told reporters. "We really don't know where our enemies might strike."

About 250 U.S. troops are based at Zamboanga's Camp Navarro, the headquarters of the Philippine Southern Command base, working on security assistance and humanitarian programs.

In early September, the government said it was intensifying security after a suspected al-Qaida member told U.S. interrogators that the terror group planned to attack unspecified targets in the Philippines.

Communist rebels, who have staged a series of attacks over the last week, also said Wednesday that Philippine military and police camps were targets for guerrilla strikes but denied government intelligence reports that they plan to attack oil depots, shopping malls and key installations in the capital.

U.S. military support helped Filipino troops decimate the Abu Sayyaf with a monthslong offensive over the summer on Basilan. But in early September, the government said it was sending reinforcements to another nearby island, Sulu, to wipe out an Abu Sayyaf faction there.

Abu Sayyaf guerrillas seized 102 hostages, including three Americans, in a yearlong kidnapping spree. In a bloody army rescue attempt, American missionary Martin Burnham and Filipino nurse Ediborah Yap were killed, while Burnham's wife, Gracia, was wounded but survived.