An Afghan immigrant has pleaded guilty to planning a suicide bomb attack on New York City's subway system.

Najibullah Zazi, 25, also admitted in a federal court in New York on Monday that he had received bomb-making and weapons training from al-Qaeda in Pakistan's Waziristan region, which borders Afghanistan.

"The plan was to conduct [a] martyrdom operation in Manhattan" around the time of the eighth anniversary of the attacks of September 11, 2001, Zazi told the court.

"To me, it meant I would sacrifice myself to bring attention to what the US military was doing to civilians in Afghanistan."

Zazi pleaded guilty to conspiracy to use weapons of mass destruction, conspiracy to commit murder in a foreign country and providing material support to al-Qaeda.

The guilty pleas - part of an agreement between prosecutors and the defence - suggest Zazi was willing to co-operate with investigators, but lawyers in the case declined to comment.

At least four other suspects have been charged in connection with the case - Zazi's father, two of Zazi's high school classmates and a New York City imam.

Zazi, who faces life in prison, said he and unnamed others travelled to Pakistan en route to Afghanistan in 2008 in order to "fight alongside [the] Taliban against the United States and its allies".

"While we were in Pakistan we were recruited by al-Qaeda. Al-Qaeda asked us to return to the United States to conduct martyrdom operations," Zazi said.

He admitted to driving from Colorado state to New York in September of 2009 with detonators and materials to build bombs, and that he threw them away when he realised the authorities had him under surveillance.

Zazi's father, Mohammed Zazi, is accused of conspiring to destroy evidence, and the imam, Ahmad Afzali, is accused of having tipped off the younger Zazi that he was under surveillance.

Mohammed Zazi and Afzali were arrested on charges of making false statements to the FBI and were let out on bail.

'Most serious threat'

Eric Holder, the US attorney-general, told reporters on Monday that "this was one of the most serious terror threats" to the US since the September 11, 2001 attacks.

"Were it not for the combined efforts of the law enforcement and intelligence communities, it could have been devastating ... and it would have been deadly," he said.

Holder used Zazi's case on Monday to say that the US criminal justice system had been invaluable in obtaining intelligence from the former airport shuttle bus driver from Denver, Colorado, as well as his guilty plea.

At a justice department news conference, Holder said Republican critics who have said the Democratic administration should try such suspects in military tribunals rather than through civilian courts, were basing their opposition on politics, not facts.

"This demonstrates that our federal criminal justice system has the ability to incapacitate terrorism suspects, to gather intelligence from terrorism suspects," Holder said.

"To take this tool out of our hands, to denigrate this tool flies in the face of facts."

Holder has opposed a bill introduced by Lindsey Graham, a Republican senator and joined by about half the senate's Republicans and a few Democrats, that would bar the trial of any of the conspirators charged in the September 11, 2001 attacks on the US, from being tried in civilian courts.

Holder wants to transfer Khalid Sheikh Mohammed, the professed mastermind of the 9/11 attacks and four other detainees, from the US military prison at the Guantanamo Bay naval base in Cuba, for trial in New York City, but his plan has run into opposition and is under review.