GUANTANAMO BAY NAVAL BASE, Cuba - A U.S. military tribunal will hear arguments Monday on whether it has the right to try Omar Khadr, a Canadian captured as a 15-year-old while fighting against American forces in Afghanistan in 2002.

Lawyers for Khadr, now 21, argue in a challenge on the hearings' agenda that the judge would be the first in western history to preside over a trial for alleged war crimes committed by a child.

"The best case scenario is the judge does the right thing and dismisses the charges on the grounds that Congress did not intend this system to apply to a minor,'' said the lead defence lawyer, U.S. navy Lt.-Cmdr. William Kuebler.

The U.S. Defence Department argues the tribunal system established by the Bush administration in 2006 is the most appropriate venue to prosecute Khadr, whom it has declared an unlawful "enemy combatant.''

From the time he was about 10 years old, Khadr travelled through Afghanistan and Pakistan with his father, an alleged al-Qaida financier, and visited with militant leaders including Osama bin Laden, according to court documents.

Khadr is accused of hurling a grenade that killed U.S. army Sgt. 1st Class Christopher Speer, a Special Forces commando, during a July 2002 firefight at an al Qaeda compound in Afghanistan.

After his capture, he acknowledged seeking to kill Americans because he was told the United States opposed Islam, military prosecutors say.

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.

As a child soldier, Khadr's lawyers say, he would have been rewarded for criminal behaviour and cannot be expected to have understood the law of armed conflict.

"The laws of war require a degree of maturity and sophistication that children simply cannot be expected to have,'' according to the motion.

Khadr's trial is scheduled for May and is on track to be the first for a detainee at the U.S. naval base in southeast Cuba, where the Pentagon's efforts to hold the first war-crimes trials since the Second World War era have been stalled by legal setbacks.

A Pentagon spokesman, navy Cmdr. Jeffrey Gordon, said that Khadr's age may be considered during the sentencing phase if he is convicted -- but it does not affect the trial.

"We believe that Khadr must be held accountable for his actions in Afghanistan after 9/11,'' Gordon said.

"International law, Canadian law, and U.S. law all hold that a person of Khadr's age at the time of the alleged offences can be tried for his crimes,'' he argued.

Prime Minister Stephen Harper has said Canada will not intervene in the case. However, other governments, rights groups and the United Nations have raised concerns.

Pascale Andreani, a spokeswoman for the French Foreign Ministry, said last month that Khadr must be given special treatment because he was a minor at the time.

France considers "all children associated with an armed conflict ... should be treated accordingly,'' she said.

Her remarks were made on the same day that the Pentagon denied a defence request to send a United Nations specialist on child soldiers to Monday's hearing.

Radhika Coomaraswamy, the UN's Special Representative for Children in Armed Conflict, raised Khadr's case during a meeting last month with a top U.S. State Department official.

She said she was concerned about the international precedent that would be set if a war crimes trial deals with alleged acts committed when Khadr was only 15.

Four human rights groups sent a letter Friday to U.S. Defence Secretary Robert Gates urging him to transfer Khadr to a court system that would consider his juvenile status, or return him to Canada for rehabilitation.

"For the past five years, the United States has ignored Khadr's rights as a child,'' said Jennifer Daskal, of Human Rights Watch, one of the four groups. "The U.S. should not make matters worse by prosecuting him before an unfair military tribunal.''

In their filing, Khadr's lawyers argue that his prosecution would violate U.S. and international laws, including a treaty ratified by the U.S. Congress in 2002 that calls for the rehabilitation of captured child soldiers.

But a spokesman for the military commissions, army Maj. Bobby Don Gifford, said conventions on child soldiers do not provide a "blank cheque'' of amnesty for war crimes.

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.