The US government suffered a stunning reversal on the legal front of its "war on terror" Monday as military judges threw out charges against a Canadian-born al-Qaeda fighter and Osama bin Laden's exdriver.

The surprise rulings on Toronto native Omar Ahmed Khadr, 20, and Salim Ahmed Hamdan threatened to torpedo the government's pursuit of Guantanamo Bay terror suspects through new-look military tribunals.

In both cases, the judges found that they had no jurisdiction to proceed with military commission trials, as neither Khadr nor Hamdan had been classified as an "unlawful enemy combatant" as required by a recent US law.

Khadr was just 15 when he was captured in Afghanistan, accused of killing a US army medic in a handgrenade attack.

Sporting a straggly beard and dressed in olive-green prison garb

and flip-flops, he stared on impassively as Colonel Peter Brownback issued his potentially far-reaching ruling.

"It was very surprising," the defendant's sister, Zaynab Khadr, told AFP in Canada. "But we were very happy to hear the news. I hope he will be released soon."

However, lawyers said that both Khadr and Hamdan will remain in legal limbo at this base in southeast Cuba along with nearly 400 other detainees rounded up or handed over to US forces since the September 11 attacks of 2001.

It was the second major victory for Hamdan, an admitted driver and bodyguard for the al-Qaeda mastermind who was born in Yemen in 1970, after he defeated the government's old tribunal process in a Supreme Court case a year ago.

Major Beth Kubala, spokeswoman for Guantanamo Bay's Office of Military Commissions (OMC), said it would be "speculative" to comment on the rulings' implications for the government.

But she told reporters: "OMC will continue to operate in a manner that's fair, transparent, open and legitimate.

"If nothing else, today's rulings highlight that the judges operate independently," Kubala added.

Defence Secretary Robert Gates brushed off a question about the dismissals, saying he had just learned about them.

"I'll have to take a look at what the implications are. But at this point I don't know enough," Gates told reporters on a trip in Kyrgyzstan.

Government prosecutors were granted a 72-hour delay to the Khadr and Hamdan rulings while they consider their options for appeal.

However, the appeals court envisioned in the Military Commissions Act (MCA), which was hurriedly passed by Congress last September following the Supreme Court ruling on Hamdan, has yet to be created.

"If the administration has any sense at all, this will be the death knell for the commissions," Jennifer Daskal of Human Rights Watch said, calling for terror suspects to be tried in US civilian courts.

Murder and other charges levied against Khadr were dismissed by Brownback. Late in the evening, Navy Captain Keith Allred threw out charges of conspiracy and material support for terrorism against Hamdan.

"Mr Hamdan is both relieved (and) still hopes he'll get a fair trial," said Hamdan's defence lawyer, Lieutenant Commander Charles Swift.

Swift said the military judges had "reaffirmed that the president is not a tribunal in and of himself."

"It was a victory for the rule of law and the law of war," he said, adding: "It shows what happens when you try to throw legislation together... in a rush."

So-called combatant status review tribunals (CSRTs) have conferred the description of "enemy combatant" on hundreds of suspects held without charge at Guantanamo.

But in a pivotal point for the judges at Monday's tribunals, none of the suspects still at the camp has been labelled "unlawful" by a CSRT, so legally they could be viewed as fighting for a legitimate state.

One time-consuming option for the government would be to convene a new round of CSRT tribunals for the Guantanamo detainees, to officially label them "unlawful" and so press ahead with prosecutions.

The only Guantanamo trial held so far has been that of 31-year-old "Australian Taliban" David Hicks, who was jailed for nine months in March after reaching a plea bargain the chief of the US commissions. He is now imprisoned in Adelaide.