The United Nations has registered its unease over the military trial of Omar Khadr, who has been in U.S. custody since age 15 and is the only Canadian being held at the Guantanamo Bay naval prison.

Radkhika Coomaraswamy, the UN special representative for the Children and Armed Conflict unit, complained Tuesday to the secretary of state's top legal adviser, John Bellinger.

"She raised her concerns about the creation of an international precedent where an individual is being tried for war crimes with regard to alleged acts committed when he was a child," said spokeswoman Laurence Gerard.

She didn't discuss details of the case against the Toronto-born Khadr, but spoke about the implications of trying him for war crimes, said Gerard, who described Coomaraswamy as an independent moral advocate for all children.

There was no word on Bellinger's response and his office did not return phone calls.

The U.S. military is determined to proceed with the case against Khadr, 21, who has been held at the military prison in Guantanamo Bay for more than five years.

He's charged with throwing the grenade that killed a U.S. medic, Sgt. Christopher Speer, in a firefight in Afghanistan in 2002.

Human-rights groups complain

Coomaraswamy has also had confidential discussions with Canada, said Gerard.

Khadr's home country has taken a hands-off approach to his case, saying he's accused of serious crimes that include spying, conspiracy, attempted murder and material support for terrorism.

Human rights groups say his military trial contravenes the Optional Protocol of the UN Convention on the Rights of the Child, which the United States has signed.

The protocol says youths under age 18 in armed conflict are entitled to special protection.

Other western countries, including Britain, have intervened to get their citizens out of Guantanamo.

Lt.-Cmdr. William Kuebler, Khadr's military defence lawyer, is in London to ask MPs and activists to put pressure on Ottawa to intervene.

60 Minutes airs Khadr footage

The United States returned nine British nationals in 2004 and 2005.

Khadr is the last remaining westerner among some 300 detainees at the U.S. naval base in southeast Cuba.

He didn't enter a plea at his arraignment earlier this month and his next hearing isn't scheduled until next February.

Defence lawyers say there's a U.S. government employee who was there when Khadr was arrested and could help clear him with evidence that was never pursued by officials.

The CBS television show 60 Minutes recently broadcast video footage of Khadr allegedly making bombs in Afghanistan.

Khadr's lawyers accused the Pentagon of leaking the video, a charge a Defence Department spokesman denied.