Omar Khadr listens to proceedings during his sentence trial at the U.S. Naval Base in Guantanamo Bay, Cuba, on Wednesday. (Janet Hamlin/Associated Press)

The United Nations representative on child soldiers says Omar Khadr should not be imprisoned in the United States but rather returned to Canada to be rehabilitated.

Radhika Coomaraswamy, the UN secretary-general's special representative for children and armed conflict, said Khadr represents the "classic child soldier narrative: recruited by unscrupulous groups to undertake actions at the bidding of adults to fight battles they barely understand."

She also said the fact that Khadr was abused by his own father exacerbates the harm done to him.

Coomaraswamy's comments are included in a letter obtained by CBC News that is dated Wednesday to the U.S. military commission at Guantanamo Bay, Cuba.

Toronto-born Khadr was 15 when he threw the grenade that killed U.S. Special Forces Sgt. Christopher Speer in Afghanistan in 2002. Since then, Khadr has been in detention at Guantanamo Bay.

At the military commission hearing in Guantanamo Bay on Monday, Khadr withdrew his previous pleas of not guilty to five war crimes charges, including murder in violation of the laws of war, attempted murder in violation of the laws of war, conspiracy, providing material support to terrorists, and spying.

Seven U.S. military officers who will decide on Khadr's sentence are currently hearing arguments from the prosecution and defence.

In her letter, Coomaraswamy said the United States has been at the forefront of the fight against the use of child soldiers.

Coomaraswamy said Khadr would like to become a medic, and said that the process of rehabilitation may help him find a new direction in his life.

"I would there urge the military commission members to consider international practice — practice supported by the US Government — that Omar Khadr should not be subject to further incarceration but that arrangement be made for him to enter a controlled rehabilitation program in Canada."

At the hearing in Cuba, Khadr's defence team is expected to argue that he is young enough that rehabilitation, rather than incarceration, would be more effective.

On Tuesday, Human Rights Watch urged the military commission to consider Khadr's former status as a child soldier in sentencing.

"The U.S. treatment of Omar Khadr has been at odds with international standards on juvenile justice and child soldiers from the very beginning," said Jo Becker, the director of children's rights advocacy for the New-York based group. "The U.S. government's failure to take into account Khadr's age should not persist at his sentencing."