A Nigerian man who tried to blow up a Detroit-bound plane on Christmas Day has confessed to training with an Al-Qa'ida bombmaker in Yemen, security officials told U.S. media yesterday.

The allegations highlight Yemen's growing centrality in global terror plots as the country's government carries out an offensive against Al-Qa'ida suspects that has reportedly killed 68 alleged militants in the past 10 days.

New details emerging about Umar Farouk Abdulmutallab suggested his abortive attack on Northwest Airlines Flight 253 was planned in Yemen by Al-Qa'ida members who even sewed an explosive device into the 23-year-old's underwear.

Law-enforcement officials told media outlets that Abdulmutallab had offered several details about his links to Al-Qa'ida and his plan to take down the flight en route from Amsterdam.

Abdulmutallab told investigators that a radical Yemeni cleric he contacted through the Internet put him in touch with an Al-Qa'ida leader living in Yemen, ABC News said.

He described spending a month at an Al-Qa'ida compound north of Yemen's capital, Sanaa, and said he was denied permission to leave the site until he completed his training alongside a Saudi Al-Qa'ida bombmaker.

U.S. counterterrorism officials said the Nigerian claimed he received specific instructions about how to carry out the attack, NBC News said.

He claimed he was told to blow up the plane and the 290 people aboard as it approached Detroit because it would produce more casualties and collateral damage on the ground if it crashed into a densely populated area.

Details emerging from Abdulmutallab's homeland suggested the young man had been a religious teenager who became radical after studying at University College London.

Nigeria's This Day newspaper reported that he relocated to Egypt and then Dubai, and while in the United Arab Emirates told his family that he was severing all contact with them.

His attitude worried his father so much that he informed the U.S. embassy in Abuja about his son's activities.

But Dutch authorities said Abdulmutallab had a valid U.S. visa when he passed through Amsterdam and his name was reviewed by U.S. authorities before he boarded the Airbus 330 travelling from the Netherlands to Detroit.

Details about when Abdulmutallab may have travelled to Yemen were still unclear, but charges filed against the Nigerian yesterday revealed new information about the device he tried to detonate.

The Justice Department alleged in charging documents that he went to the bathroom before the plane began its

final descent, spending 20 minutes away from his seat before returning and saying he had an upset stomach.

"He pulled a blanket over himself. Passengers then heard popping noises similar to firecrackers, smelled an odor and observed Abdulmutallab's pants leg and the wall of the airplane on fire," the affidavit said.

"One flight attendant ... stated that she asked Abdulmutallab what he had in his pocket and he replied 'explosive device.'

"A passenger stated that he observed Abdulmutallab holding what appeared to be a partially melted syringe, which was smoking. The passenger, Dutchman Jasper Schuringa, took the syringe from Abdulmutallab, shook it to stop it from smoking and threw it to the floor of the aircraft," the affidavit added.

Remnants of the syringe had been recovered and were believed to be part of the explosive device, the document said.

The affidavit described the device as containing PETN, also known as pentaerythritol, "a high explosive" that is similar to nitroglycerin.

The substance was used in 2001 by Richard Reid, the so-called "shoe bomber" whose attempt to blow up a trans-atlantic flight also was thwarted.

the new york times contributed to this report