A bomb hidden in a backpack ripped through an airport in the southern Philippines today, killing at least 21 people, including an American missionary, and wounding more than 100.

No one claimed responsibility for the attack in Davao, the Philippines' second-largest city, but security officials blamed a pair of Muslim militant groups waging an armed campaign to establish a separate state in the southern part of the country. The officials said they were holding five people for questioning.

Officials said the bombing was partly the work of the Moro Islamic Liberation Front (MILF), a separatist group believed to be responsible for several recent attacks, including a fatal car bombing last month at another airport in the southern Philippines. The group, which has been engaged in a sputtering peace process with the Philippine government, denied it was behind today's attack.

Some security officials said Moro members had carried out the attack in cooperation with rebels from the Abu Sayyaf, a smaller separatist organization linked by the United States and other governments to al Qaeda.

President Gloria Macapagal Arroyo called today's bombing a "brazen act of terrorism which shall not go unpunished" and convened an emergency meeting of her cabinet oversight committee on security.

"Her orders were for the Philippine National Police and the armed forces of the Philippines to hunt down the bombers and to see to it that even suspected lairs of possible terrorists be cleared," said a spokesman for the president, Rigoberto Tiglao.

Arroyo's government also directed that security be heightened around vital installations such as power stations and airports. Flights to and from Davao were suspended.

The blast occurred at 5:20 p.m. about 50 yards from the main terminal, in an open-air shelter where a crowd had gathered to welcome passengers who had just arrived on a Cebu Pacific Air flight from Manila, the capital, according to police. Edgar Aglipay, the deputy national police chief, said at least one assailant had concealed explosives in a small backpack left beneath a bench. The blast blew off sections of the corrugated metal roof, scattering debris into the adjacent parking lot.

"This was the worst possible time for something like this to happen. This is when three planes come in from Manila in rapid succession and the shed is filled with people waiting to meet their relatives," said the Rev. Jack Walsh, an American priest who works near the airport.

He said he heard the explosion from his office but initially thought it was the sound of passing truck traffic or heavy equipment in the nearby port.

Among the victims was the Rev. William P. Hyde, an American missionary and resident of the Davao area, who had come to the airport to pick up an American missionary family. Manuel Tan Jr. of the Davao Medical Center said Hyde, a member of the Southern Baptist Convention, died in surgery.

Three members of the missionary family, Barbara Wallis Stevens, 33, her 10-month-old son, Nathan, and 4-year-old daughter, Sarah, were treated for injuries at the Davao Doctors Hospital, medical staff said.

Around the same time, a second bomb exploded at the main bus terminal in Davao, but no one was seriously injured, police said. An hour later, another explosion rocked a government building in the town of Tagum, north of Davao, injuring two people, they said. Security officials said they were not sure if all the bombings were linked.

According to security sources, government officials suspect the Davao attack was carried out by a group called Balik Islam, which draws on bomb experts from both the MILF and the Abu Sayyaf and seeks to create chaos in the southern Philippines through economic sabotage. One source said Balik Islam is headed by an Abu Sayyaf commander named Hamsiraji Sali.

Sali has told Philippine reporters that his group began receiving financial support from Iraq after demonstrating that it could embarrass the Manila government. Security experts, however, have not been able to confirm Sali's claim of Iraqi support.

The attack today came as tension between the Philippine government and separatists in the largely Muslim south had escalated in recent weeks. Last week, the MILF blew up power pylons, plunging most of the southern island of Mindanao into darkness, in response to an offensive by government forces that captured a key rebel enclave. The MILF was also blamed by government officials for a car bombing last month at the airport in the southern city of Cotabato that killed one person and an explosion in a market in Kabacan that left another dead. The MILF has denied involvement in these blasts.

Military intelligence officials said the MILF has carried out about 30 bomb attacks in the south since January 2001, when Fathur Rahman Ghozi, an Islamic militant from Indonesia, was arrested in Manila. Ghozi, a suspected member of the regional Jemaah Islamiah radical network, helped train militants in bombmaking techniques at Camp Abubakar, an MILF camp in Mindanao that was shut in 2000, intelligence officials said.

Arroyo has stepped up her government's campaign against the Abu Sayyaf, announcing Friday that she would give the Philippine military 90 days to eradicate the group. U.S. Special Forces are in Mindanao to train Philippine military units in counterterrorism tactics in the area of Zamboanga, 220 miles west of Davao.

Special correspondent Froilan Gallardo in Cagayan de Oro City, Philippines, contributed to this report.

Medical workers assist a boy injured when explosives hidden in a backpack blew up at an airport in the city of Davao.