Manila, Philippines ? A bomb planted inside a backpack Tuesday ripped through an airport terminal in the southern Philippines, killing at least 21 people — including an American missionary — and injuring 145 in the nation’s worst terrorist attack in three years.

The blast comes at a time of heightened debate over the role of U.S. troops in the war on terror in the Philippines, where Muslim insurgents have battled the government for decades with attacks, bombings and kidnappings.

President Gloria Macapagal Arroyo, who invited U.S. troops to help train Filipino soldiers in counterterrorism, said the bombing at Davao airport on Mindanao island was “a brazen act of terrorism which shall not go unpunished.”

Despite Tuesday’s attack, Arroyo today ruled out any combat role for U.S. troops in the southern Philippines.

No one claimed responsibility for the blast, but Arroyo said “several men” were detained. The military has blamed Moro Islamic Liberation Front rebels for a string of attacks, including a car bombing at nearby Cotabato airport last month that killed one man.

Eid Kabalu, spokesman for the rebel group, which has been fighting for Muslim self-rule in the predominantly Roman Catholic Philippines for more than three decades, denied his group was responsible.

Police said the bomb was hidden in a backpack planted in the airport’s waiting area. The blast was heard three miles away; some of the debris landed on the tarmac 100 yards away.

The Southern Baptist Convention’s International Mission Board in Richmond, Va., confirmed that missionary William P. Hyde, 59, of Cedar Rapids, Iowa, died in surgery from head and leg injuries.

Hyde, a former music teacher, had been a missionary since 1978. He and his wife, Lyn, have two grown sons, one of whom is a missionary.

David Miller, pastor of Northbrook Baptist Church in Cedar Rapids, said the Hydes had been close friends of Martin and Gracia Burnham, American missionaries who were kidnapped in 2001 by another Muslim extremist group, Abu Sayyaf. Martin Burnham was killed during a rescue operation in June 2002, and his wife was wounded. Gracia Burnham now lives in Rose Hill, Kan.