Omar Khadr is seen in this undated family portrait. REUTERS/Handout/Files

MIAMI (Reuters) - A Canadian captured in Afghanistan at age 15 can be tried for murder in the Guantanamo war crimes court, a U.S. military judge ruled in rejecting claims that he was a child soldier who should be rehabilitated rather than prosecuted.

Canadian prisoner Omar Khadr, now 21, is charged in the Guantanamo court with throwing a grenade that killed a U.S. soldier during a firefight at a suspected al Qaeda compound in Afghanistan in 2002.

His military lawyer, Lt. Cmdr. William Kuebler, had argued in February hearings at the Guantanamo naval base that Khadr was a child soldier illegally conscripted by his father, an al Qaeda financier. He urged the judge to drop the charges, which carry a maximum penalty of life in prison.

The judge, Army Col. Peter Brownback, issued a ruling on Wednesday agreeing with prosecutors’ position that the law authorizing the Guantanamo trials contained no minimum age.

Brownback’s ruling clears the way for Khadr to be tried in the special tribunals created by the Bush administration to try non-U.S. captives it considers “unlawful enemy combatants” outside the regular civilian and military courts.

Kuebler called the ruling “an embarrassment to the United States” and said Canada would share in the embarrassment if it allows its citizen to be tried at Guantanamo. He said Khadr would be the first child soldier tried for war crimes in modern history.

The United States and Canada have ratified an international treaty, the Child Soldier Protocol, that outlaws recruitment of combatants under age 18 and requires governments to help child soldiers recover and reintegrate into society.

It does not specifically bar prosecution of child soldiers but says they should not be subjected to cruel, inhuman or degrading treatment or punishment, and that they cannot be subjected to life imprisonment without possibility of release.

Khadr, who was shot twice in the back by U.S. soldiers during the battle that led to his capture, is charged with murder, attempted murder, conspiring with al Qaeda, providing material support for terrorism and spying on U.S. military convoys in Afghanistan.