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 148 in the nation's worst terrorist attack in three years.

Authorities said Wednesday that five Moro Islamic Liberation Front members had been arrested in connection with the blast and were being questioned. No one has claimed responsibility for the bombing.

The blast at Davao airport 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.

Susan Madrid, spokeswoman for the Davao civil defense, said 21 people were killed, including an American missionary, and 148 were injured. She said two of the injured died overnight. There were no details on the latest fatalities, but the earlier deaths included a boy, a girl, 10 men and seven women.

Three Americans -- a Southern Baptist missionary and her two young children -- were among the wounded.

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."

Arroyo arrived in Davao Wednesday to visit the bombing site. A high-ranking delegation led by the nation's police chief and the interior secretary inspected the damaged airport terminal, where workers used brushes to clean blood stains from the ground.

President Bush condemned the attack as a "wanton terrorist act" and sent condolences to the people of the Philippines, his press secretary Ari Fleischer said.

"The president notes that the bombing underscores the seriousness of the terrorist threat in the southern Philippines, and he emphasizes that the Philippines have been a stalwart partner of the United States in the war against terror," Fleischer said.

Interior Secretary Jose Lina said that the five suspects were undergoing questioning. "There is a basis for their arrest and there is evidence on them," he said, without elaborating.

The military has blamed Moro rebels for a string of attacks, including a car bombing at nearby Cotabato airport last month that killed one man.

U.S. special forces are training Philippine soldiers in counter-terrorism tactics in the city of Zamboanga, about 250 miles west of the scene of Tuesday's attack.

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. He condemned the attack and said the group was ready to cooperate in an investigation.

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.

Hyde had gone to the airport to meet American missionaries Barbara Wallis Stevens and Mark Stevens and their family, who were had just arrived from Manila when the bomb went off.

"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."

Her 10-month-old son Nathan was hit by shrapnel and sustained an injury to his liver, but a doctor at Davao Medical Center said he was out of danger. Her daughter was also injured but released after treatment. The family has lived in Davao, on southern Mindanao island, for five years doing missionary work with indigenous 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.

U.S. defense officials announced last month 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.