WASHINGTON - The U.S. Defence Department is pressing ahead this week with Canadian Omar Khadr's terror trial despite major questions about the future of military commissions at Guantanamo Bay.

For the third time in nearly two years, the Pentagon is trying to make headway on the case against Khadr, who's charged with murdering an American soldier in a July 2002 firefight in Afghanistan.

The system for trying foreign terror suspects set up by President George W. Bush after the 2001 attacks in the United States has been plagued from the start by fundamental legal problems that led to an overhaul last year.

And Khadr, 21, is still among the first to face the new system, despite the capture of several so-called "high value'' detainees who allegedly had a direct hand in the 9-11 attacks.

"What they've got in Omar's case that they don't have in others, and I mean this respectfully, is a dead body,'' said Muneer Ahmad, who once served on Khadr's legal team.

"It looks like a crime. In the other cases, they have a loose conspiracy theory.''

And prosecutors, said Ahmad, are intent on substantiating a system that has been struck down once by the U.S. Supreme Court and widely attacked by critics who say it fails to adhere to the rule of law.

"I think the prosecution is trying to get out in front of it all and say: `Look, this is a legitimate system.' Because if they can't figure out how to legitimize this system, they really are sunk.''

The Supreme Court could yet throw a wrench in the works when it rules in coming months on whether the prisoners should be able to challenge their detention in regular U.S. courts.

The Bush administration has already talked about wanting to close the Guantanamo prison camp, a target of worldwide condemnation, but that's not expected to happen any time soon.

Meantime, the Pentagon staunchly defends the tribunal system that is eventually supposed to try about 80 of some 350 prisoners currently held at the U.S. naval base in Cuba, most picked up on battlefields in Afghanistan and Pakistan.

Only four have been charged under the new Military Commissions Act passed by Congress last year.

One case, that of Australian David Hicks, ended in a plea bargain. The trials haven't started yet for the three others, including Khadr.

The Canadian, who's been in U.S. custody since he was 15 years old, is charged with throwing the hand grenade that killed Sgt. First Class Christopher Speer.

Other charges include attempted murder, conspiracy, providing material support for terrorism and spying.

Khadr's latest hearing, scheduled for Thursday, comes amid an upset in prosecution ranks and a feud among his defence lawyers.

Former chief prosecutor Col. Moe Davis abruptly resigned last month, saying he'd been pressured to rely on classified evidence that would ensure many of the trials took place behind closed doors.

Some Pentagon officials didn't share his view of the need for transparency and constant media coverage, he said.

On the defence side, Canadian lawyer Dennis Edney has been barred from Thursday's arraignment after complaining the lead U.S. military attorney, Lt.-Cmdr. William Kuebler, isn't ready for trial because he spent months on a blitz in Canada to gather support for sending Khadr home.

Khadr, says Edney, doesn't want to be represented by a military lawyer and his wishes are being violated.

The question of Khadr's representation will likely arise at the hearing, said Kuebler.

He also expects the trial judge, Col. Peter Brownback, will hear evidence on whether Khadr is an "unlawful'' enemy combatant who didn't have the right to fight with the Taliban against U.S. soldiers in Afghanistan.

That designation is required by Congress in order for Khadr's trial to proceed.