GUANTANAMO BAY NAVAL BASE, Cuba - A young Canadian terror suspect was shot twice in the back by U.S. forces during a 2002 firefight in Afghanistan after he allegedly killed an American commando, according to a U.S. fighter's eyewitness account revealed Monday.

Omar Khadr, who was 15 at the time, already had been injured by shrapnel in his chest and was facing away from the fight when he was shot inside an al Qaeda compound, according to the unidentified U.S. fighter.

Defence lawyers said the new details would help them make a case that Khadr should not be tried before a military tribunal because his alleged offences occurred in a combat setting, and therefore should not be considered war crimes committed by an irregular "enemy combatant.''

The Toronto-born Khadr, now 21, is charged with hurling a grenade that killed army Sgt. 1st Class Christopher Speer.

The eyewitness also says a second enemy fighter was alive inside the compound when he entered, casting doubt, defence lawyers say, on the government's conclusion that Khadr hurled the grenade.

"It does raise questions as to basic facts of the government's case,'' said the defence lawyer, navy Lt. Cmdr. William Kuebler.

The witness account was included in motions handed out to reporters at a pretrial hearing for Khadr, whose case is on track to become the first to go to trial at Guantanamo.

At the hearing, defence lawyers urged the judge to drop the charges against Khadr, arguing that prosecuting somebody so young at the time of the alleged offence would violate international treaties protecting child soldiers.

The judge, army Col. Peter Brownback, did not immediately issue a ruling. The Pentagon's case against Khadr, twice derailed by legal challenges, is scheduled for trial in May before the first U.S. war-crime tribunals since the Second World War era.

As a child, Khadr travelled through Afghanistan and Pakistan and met terrorist leaders including Osama bin Laden through his father, Ahmad Said al-Khadr, a suspected al Qaeda financier who was killed in Pakistan in 2003. He later attended terrorist training camps and planted landmines targeting American convoys, according to court documents.

Lawyers said he was not old enough to think differently from his family.

"He is a victim of al Qaeda, not a member of al Qaeda,'' Kuebler told the judge.

Military prosecutors rejected arguments that Congress did not intend for the tribunals authorized by a 2006 law to apply to minors. Protections afforded child soldiers under international law do not apply to Khadr, they said.

"Any immunities under international law have been abrogated by U.S. prosecution of him,'' Andy Oldham, a Justice Department prosecutor, told the judge.

Khadr was captured in Afghanistan after the firefight and has been held at Guantanamo for more than five years. During the hearing, he did not speak and appeared bored, drawing and fidgeting during the arguments.

Khadr faces a maximum sentence of life in prison. He has been charged with war crimes including murder, conspiracy, and providing material support to terrorism.

Defence lawyers also argued the law authorizing U.S. military tribunals should not apply to Khadr because the alleged offences occurred before the court was created by the U.S. Congress in 2006, after the U.S. Supreme Court threw out the rules for a previous court system.

Prosecutors, however, said Congress clearly meant for the tribunals to prosecute terrorism suspects who were involved in or supported the Sept. 11, 2001, attacks.

Human rights observers said the continuing emergence of the new details about the firefight reflected a closed process at Guantanamo.

At Khadr's last pretrial hearing in November, his lawyers said they learned only a few days earlier about the existence of an eyewitness whose account of the firefight could help them make a case that he was not an "unlawful enemy combatant'' and disqualify him for trial. Kuebler said it was not the same witness.

"It emphasizes that we need a truly independent, transparent process,'' said Jumana Musa, advocacy director of London-based Amnesty International.

A Pentagon spokesman, navy Cmdr. Jeffrey Gordon, disputed the contention that the Guantanamo trials are not open, noting the attendance of journalists and independent observers.

The military has charged four of about 275 detainees now held at Guantanamo on suspicion of terrorism or links to al Qaeda or the Taliban. Authorities have said they plan to prosecute about 80 prisoners, including the alleged architect of the Sept. 11 attacks and 14 other so-called "high-value'' detainees.