MANILA, Philippines -- A bomb planted inside a backpack ripped through an airport terminal in the southern Philippines on Tuesday, killing at least 21 people -- including an American missionary -- and injuring about 150 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.

Three Americans -- a Southern Baptist missionary and her two young children -- were among the wounded. Many of the injured were in serious condition, and officials feared the death toll could rise. The dead included a boy, a girl, 10 men and seven women.

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

No one claimed responsibility for the blast, but Arroyo said "several men" were detained.

Hidden in backpack

Police said the bomb was hidden inside a backpack planted in the middle of 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.

"I just heard it explode to my side," said Barbara Wallis Stevens, 33, of Willard, Mo., who was slightly wounded. "I was carrying my infant son so I grabbed my daughter and picked her up and ran away. I was afraid there could be more bombs."

She said 10-month-old Nathan was hit by shrapnel in the liver. Her daughter, Sarah, was also injured but released after treatment. The family has lived in Davao for five years doing missionary work with local tribes.

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 in Cambodia.

David Miller, pastor of Northbrook Baptist Church in Cedar Rapids, called Hyde "kind of the teddy-bear type -- kind, gentle and always smiling."

"The irony of a man that sweet and kind being killed in an act of terror and hatred is just really sad," Miller said.

"They knew that it was dangerous over there," he added. "They were on our prayer sheet week by week for their safety."

Miller 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.

In a separate incident Tuesday, an explosion in Tagum, about 20 miles north of Davao, injured three people, said military spokesman Lt. Col. Daniel Lucero. The military suspected Moro Islamic Liberation Front guerrillas in that attack.

Arroyo has proposed a peace agreement with the rebel group, but the rebels said they will not negotiate unless government troops withdraw from areas they captured last month.

Kabalu, the rebel spokesman, said the explosions "might be a plot of the military" to extend the planned U.S.-Philippines exercises to Mindanao island.

Philippine and U.S. officials are in disagreement over the role U.S. troops will play in training Filipino soldiers to fight Abu Sayyaf, which is notorious for kidnappings and killings and has been linked to the al-Qaida terror network.

Some 1,200 American troops, including 160 special forces troops, were sent to Philippines last year in what officials said was a mission to "train, advise and assist" Filipino forces battling Abu Sayyaf guerrillas on Basilan island.

Last month, U.S. defense officials announced they had an agreement to deploy more than 1,000 U.S. troops in an effort to rout the remaining Abu Sayyaf forces from the nearby island of Jolo. But the offensive was put on hold after Pentagon officials described the deployment as "joint operations."

That wording caused an uproar in the Philippines. Newspapers, lawmakers and left-wing groups accused the Manila government of violating the constitution that bars foreign troops from combat. Manila repeatedly denied there would be a U.S. combat role, saying Americans were coming for training exercises.

Five almost simultaneous bomb blasts in Manila on Dec. 30, 2000, killed 22 people and wounded more than 100 others. In October last year, one U.S. soldier and two other people were killed in a bombing attack in Zamboanga on Mindanao island.