JOLO, Philippines ? A powerful explosion in a karaoke bar near a Philippine army camp wounded at least 20 people 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.

Security is a top concern during the exercises because of the presence of al-Qaida-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.

The explosion occurred on the day about 250 American troops were welcomed by local officials in a ceremony on Jolo. They are to take part in “Balikatan,” an annual joint war exercise between American and Filipino troops that has focused in recent years on counterterrorism.