Lawyers for Omar Khadr called on U.S. authorities Monday to dismiss all charges against the Canadian man, saying a military commission does not have jurisdiction to try him because he was a child soldier at the time of his alleged offences.

Speaking at a hearing at the U.S. naval base in Guantanamo Bay, Cuba, the Toronto-born Khadr's lawyers argued that the commission was not set up to be a juvenile court and only has jurisdiction to try adults.

Khadr, who is now 21, has been in custody at Guantanamo Bay since 2002. He was arrested at age 15 during a firefight in Afghanistan with U.S. troops.

He is accused of murder in the death of U.S. medic Sgt. First Class Christopher J. Speer. Khadr is also charged with spying, conspiracy and supporting terrorism.

Khadr's military defence lawyer Lt.-Cmdr. Bill Kuebler noted on Monday that the U.S. has signed on to a UN protocol that states fighters under 18 years of age are to be considered as child soldiers.

The protocol says soldiers of that age are not old enough to make decisions and should be treated as victims in need of rehabilitation rather than perpetrators.

Defence lawyer Rebecca Snyder also told the tribunal that Khadr is not eligible to be tried for murder as a war crime because the alleged offence occurred during a firefight under traditional laws of war.

"Soldiers are not protected targets," she argued. "That is part of what war is about, killing soldiers."

But the lead prosecutor, Marine Corps Maj. Jeffrey Groharing, said Khadr should be prosecuted at the special U.S. tribunal because he conducted surveillance in civilian clothing and lived with women and children at a compound where the firefight occurred.

"The accused and the terrorists he was working with did not belong to a legitimate army. They belonged to al-Qaeda," Groharing said.

Defence lawyers also argued that the military commission process itself is illegitimate because it was set up by Congress at the urging of the White House in 2006, four years after the alleged crimes were committed.

Prosecutors said Congress clearly meant for the tribunals to prosecute those who were involved in or supported the Sept. 11, 2001, hijackings.

The three-hour hearing focused on only a couple arguments among the 15 different motions Khadr's defence team prepared.

The judge, Col. Peter Brownback, did not immediately issue a ruling.

Khadr, with a neatly trimmed beard and looking fit, was wearing a white prisoner uniform, an indication he is a co-operative prisoner, CBC reporter Bill Gillespie said.

Khadr seemed uninterested in the legal arguments, appearing to be praying or reading during the proceedings.

With files from the Associated Press