A U.S. military judge has rejected a bid by defence lawyers to have charges against Canadian Omar Khadr dismissed because he was only 15 at the time he is alleged to have committed offences.

Col. Peter Brownback issued the ruling Wednesday at the U.S. military prison in Guantanamo Bay, Cuba, where Khadr has been held for more than five years.

Khadr's lawyer, U.S. Lt.-Cmdr. William Kuebler, argued three months ago that prosecuting Khadr contravenes international law, including the Child Soldier Protocol signed by the United States.

The Toronto-born Khadr, now 21, faces life in prison amid allegations he threw a hand grenade that killed a U.S. soldier in Afghanistan during a July 2002 firefight.

He was brought, badly wounded, to the U.S. naval base in Guantanamo Bay in October 2002. He is scheduled to be tried by a military commission on charges that include murder, and is one of only a handful of Guantanamo Bay prisoners to face charges.

Kuebler, who appeared before a Canadian parliamentary committee to plead Khadr's case earlier this week, called Wednesday's ruling an embarrassment for the United States.