"Forty-two recruits died in the attack and others are hospitalised, some are in critical condition," military spokesman Major General Shaukat Sultan told AFP.

The spokesman condemned the bombing, the deadliest since the Pakistan army was deployed in the tribal region about five years ago to hunt down Al-Qaeda militants.

He described it a terrorist attack and warned the death toll could rise. Other security officials said 20 people were wounded.

Sultan said the investigations were ongoing, but the attack appeared to be an act by militants whose training school was destroyed by the military raid at in Bajaur last week.

"We strongly suspect the attack on the army center was done by the people trained in Bajaur in the madrassa run by Al-Qaeda facilitators Maulvi Liaqat and Maulvi Faqir," Sultan said.

Suspicion has fallen on Tehreek-e-Nifaz-e-Shariat Mohammadi (TNSM), a group which has widespread presence in the area and is known for its links to Afghanistan's Taliban and Al-Qaeda.

Both Faqir and Liaqat, who was among 80 militants killed in the airstrike, were members of TNSM group President Pervez Musharraf banned in January 2002 for sending some 10,000 armed tribesmen into Afghanistan to fight the US-led invasion.

Interior Minister Aftab Sherpao said the attack was a suicide bombing and vowed there would be no let up in the anti-terror hunt launched by the key US ally, Pakistan.

An interior ministry official said the bomber targetted recruits on a parade ground next to the regiment's main training center.

"The bomber was clad in a shawl and he blew himself up in the middle of the assembly causing large scale casualties," the official told AFP.

Hidayatullah Khan, who saw the explosion from his newspaper stall just a few hundred metres (yards) from the parade ground, described the grisly aftermath.

"Human limbs, army caps and shoes were scattered all around the blood-splashed ground," he said.

A local official said the military cordoned off the area around the base and put up barricades to prevent people leaving or entering the tribal district, some 160 kilometres (100 miles) northwest of the capital Islamabad.

He said investigators have found a torso which appeared to be that of the bomber and were examining it.

The area had until now not been touched by the fighting between Al-Qaeda-backed militants and the Pakistani military, but it is only 80 kilometres southeast of Bajaur.

The government alleged Al-Qaeda used the Islamic school or madrassa to train suicide bombers, but hardline Islamic parties accused the authorities of killing innocent students.

Tens of thousands of armed tribesmen and students protested against the airstrike, shouting slogans against Musharraf, and burning effigies of him.

Hours after the bombing an unknown caller claimed that Pakistani Taliban carried out Wednesday's suicide attack.

The man, who refused to give his name in a telephone call to a prominent journalist Rahimullah Yousafzai of local daily The News, vowed that he would himself be the next suicide bomber.

The caller said the attack was launched by Pakistani Taliban, identifying the group's leader as Abu Kalim Mohammad Ansari, Yousafzai told AFP.

The Pakistani military said the Bajaur madrassa was frequently visited by Al-Qaeda number two, Ayman Al-Zawahiri, but that he was not there at the time of the airstrike. It denied the United States was involved in the attack.

The semi-autonomous tribal regions of northwest Pakistan are a hotbed of opposition to Musharraf's alliance with the US, and are seen as strongly supportive of the Taliban and Al-Qaeda.

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