A powerful explosion in a karaoke bar near a Philippine army camp killed one person and wounded about 20 Saturday on southern Jolo island, where American troops are staying for joint war exercises.

A U.S. military spokesman, Lt. Col. Mark Zimmer, said there were no American casualties and that the explosion would not hamper the two-week joint counterterrorism maneuvers that are to start Monday and focus on humanitarian projects.

"The explosion last night was caused by a bomb made of ammonium nitrate," Brig. Gen. Alexander Aleo, the top military officer on Jolo island, said Sunday. He said a Philippine driver working on contract for U.S. troops was killed and about 20 other people were wounded.

Security is a top concern during the exercises because of the presence of Al Qaeda-linked Abu Sayyaf guerrillas on Jolo, about 580 miles south of Manila. The guerrillas have kidnapped Americans in the past and threatened to attack U.S. troops in the country.

About 250 American troops are to take part in "Balikatan," an annual joint war exercise between American and Filipino troops that has focused in recent years on counterterrorism.

The exercises this year are being held simultaneously in Manila and a number of other venues, including on Jolo, where Americans would mainly provide dental treatment for poor villagers, construct classrooms and give away medicines and books, officials said.

Witnesses said the explosion Saturday was so powerful that it caused part of the bar's roof to collapse and portions of its concrete wall to crumble.

Security for the Americans and opposition to the war drills by Muslim villagers have been nagging concerns on Jolo, which has a surfeit of unlicensed guns, frequent bloodshed and a bitter history with American forces.

Muslim activists still cite a violent U.S. campaign to quell insurrectionists resisting U.S. rule in the early 1900s.

The Philippine military has been struggling to wipe out the Abu Sayyaf — at times with covert U.S. non-combat assistance — on Jolo, but a few hundred mountain-based guerrillas continue to threaten the impoverished island and nearby regions.