A Nigerian who tried to blow up an international flight near Detroit in the US on behalf of al Qaida is starting a life sentence without parole.

The mandatory punishment for Umar Farouk Abdulmutallab was never in doubt after he pleaded guilty in October. The 25-year-old former London student said the bomb in his underwear was a "blessed weapon" to avenge poorly-treated Muslims worldwide.

The bomb failed to fully detonate on board the Amsterdam-to-Detroit flight but caused a brief fire which burned Abdulmutallab.

He admitted afterwards that the attack was inspired by Anwar al-Awlaki, a radical American-born cleric and leading al Qaida figure who was killed by a US drone strike last autumn.

Federal Judge Nancy Edmunds announced the sentence in a crowded courtroom which included some passengers from Northwest Airlines Flight 253. Earlier, four passengers and a crew member told Judge Edmunds that the event changed their lives forever.

Abdulmutallab looked disinterested during their remarks - he rarely looked up while seated just a few feet away, wearing a white skull cap and an oversized prison T-shirt.

He "has never expressed doubt or regret or remorse about his mission", the judge said. "In contrast, he sees that mission as divinely inspired and a continuing mission."

Life in prison is a "just punishment for what he has done", she added. "The defendant poses a significant ongoing threat to the safety of American citizens everywhere."

Abdulmutallab, who was educated in Europe and is the son of a wealthy Nigerian banker, told the government that he trained in Yemen under the eye of al-Awlaki.

"Mujahideen are proud to kill in the name of God. And that is exactly what God told us to do in the Koran... Today is a day of victory," he said in court.