NEW YORK: Afghan immigrant and self-confessed Al-Qaeda agent Najibullah Zazi on Tuesday faced the prospect of a life behind bars, after pleading guilty to a plot to blow up New York's subway system.

Zazi, 25, on Monday pleaded guilty to three charges: conspiracy to use weapons of mass destruction, conspiracy to commit murder in a foreign country and providing material support to the Al-Qaeda Islamist network.

He told the federal court in Brooklyn that his plans to blow up New York targets just after the eighth anniversary of the 9/11 attacks included "mortal operations" in the underground train system.

The guilty plea also handed President Barack Obama's administration ammunition of sorts to fend off Republican criticism and argue that the civilian court system is an appropriate forum to prosecute terror suspects.

Describing the plot as one of the most "serious" terrorist threats since September 11, 2001, Attorney General Eric Holder criticized those who "denigrate" the civilian justice system because they oppose a plan to try the alleged 9/11 plotters attack before US courts.

"In this case as it has been in so many other ones, the criminal justice system has proved to be an invaluable weapon for incapacitating plots and obtaining intelligence," he told a news conference.

He described the criticism of those who want to try Zazi and others in military commissions -- which would afford defendants fewer constitutional protections -- as "more about politics."

Zazi, a legal US resident who previously claimed he was innocent, said he was ready to "sacrifice" himself "to draw the attention to what the United States is doing in Afghanistan."

He faces a maximum sentence of life in prison for the first two charges and 15 years for the third. However, his plea appears to be part of a bargain in which he could receive more lenient punishment in return for cooperating with investigators.

Sentencing was set for June 25.

"This was one of the most serious terrorist threats to our nation since September 11th, 2001," Holder said. "It could have been devastating."

Zazi was arrested September 20 in Denver, where he worked as an airport shuttle bus driver. After being brought to New York, he initially pleaded not guilty to the alleged attempt to cause carnage in the city.

But Monday's guilty plea backed allegations made by US prosecutors, who described Zazi as an Al-Qaeda-linked militant attempting to use "weapons of mass destruction" here.

Prosecutors outlined how Zazi and others traveled to Afghanistan in August 2008 with the intention of joining the Taliban. Instead, they were recruited by Al-Qaeda in Peshawar, Pakistan, and trained to use weapons and explosives.

"During the training, Al-Qaeda leaders asked Zazi and others to return to the United States and conduct suicide operations. They agreed," the Department of Justice said in a statement.

Returning to Colorado, Zazi shopped for large quantities of beauty supply chemicals to concoct explosives in a hotel room, before driving to New York.

He was planning to set off bombs either on September 14, 15 or 16, 2009, the Department of Justice says, but Zazi left after receiving a tip-off that he was being watched by federal agents.

He admitted in court that he also "took trips to New York City to plan" the attacks. However, he said that instead of carrying out the plot, "we threw away the explosive material."

Federal Bureau of Investigation (FBI) deputy director John Pistole described the plea as a "significant milestone in this ongoing and complex case" that posed a substantial threat to US security.

"This case has given us all greater insight into the evolving nature of the terrorist threat we face today," he said in a statement.

"It shows how individuals here in the US with the intent to do us harm can acquire the means to do so and then take steps to carry out an attack."

Zazi's father, Mohammed Walis Zazi, who is charged with conspiring to destroy evidence in the case, was freed on bail last week. A New York imam accused of lying to the FBI about the matter is also free on bail.

Two other New York men were arrested in January and named as associates of Zazi.

The Justice Department said they were continuing to investigate the case, but that there was no imminent danger from the plot.

"I think that the American people are safe with regards to this plot and the investigation is nevertheless ongoing," Holder said.