In this Pentagon-approved photograph of a sketch by artist Janet Hamlin, Omar Khadr listens to closing arguments on Saturday in Guantanamo Bay, Cuba. (Canadian Press)

A U.S. military panel in Guantanamo Bay, Cuba, says Canadian-born Omar Khadr should serve 40 years in prison for war crimes, a symbolic decision because a pre-trial plea deal caps his sentence at eight years.

Khadr pleaded guilty in a plea bargain last Monday to five war crimes charges brought by the U.S. military, including killing Sgt. 1st Class Christopher Speer in Afghanistan in July 2002 when he was 15 years old.

The sentencing panel was not told about the plea deal before it deliberated nearly nine hours over two days at the U.S. naval base at Guantanamo Bay where Khadr has already served eight years in detention.

Toronto-born Khadr, 24, will have to serve one year in U.S. custody before he is eligible to apply for transfer to Canada.

The panel of three female and four male U.S. military officers returned its decision Sunday afternoon after asking the military judge to replay testimony from defence witness Capt. Patrick McCarthy, who interacted extensively with Khadr and found him friendly, non-radical and believed he could be rehabilitated.

The CBC's Laurie Graham, who is in Guantanamo, said the sentence was much longer than the 25-year sentence the prosecution had asked for, and "clearly this panel wanted to send a message."

On hearing the sentence, Khadr's Canadian lawyer Dennis Edney said justice was not served in this case, which he said was the first prosecution of a juvenile for war crimes in six decades.

"The fact that the trial of a child soldier, Omar Khadr, has ended with a guilty plea in exchange for his eventual release to Canada does not change the fact that fundamental principles of law and due process were long since abandoned in Omar's case," he said in a statement.

"In exchange for repatriation, Omar was required to sign an admission of facts which was stunning in its false portrayal of him."

Diplomatic notes between Washington and Ottawa released Sunday evening by the Pentagon show the U.S. will support his prison transfer after one additional year in custody at Guantanamo, and Canada will look on it "favourably."

The federal government hasn't said whether it will repatriate Khadr and reiterated Sunday afternoon that it didn't interfere in this judicial matter that's between Khadr and the United States.

Foreign Affairs spokeswoman Melissa Lantsman added that Khadr will be treated like any other Canadian who applies for a prison transfer.

"The decision on any potential transfer will be made in accordance with current law," she told The Canadian Press. "No decision can be made until an application is received."

The comments were made before the diplomatic notes were disclosed showing that Canada was aware of the plea deal.

Widow welcomes decision

The long sentence was welcomed by Speer's widow.

"Today is a huge victory for my family … and for hundreds of families out there," said Tabitha Speer, who earlier in the week blasted Khadr as a killer. "The plea, for us, is final."

The prosecution said the panel's desired sentence sent a message both to Khadr and to terrorists around the world, but defended the plea deal as the best way to bring a definitive end to the case.

"What this represents to the government is the certainty of a conviction, it represents the end of a case that spans five years," said Capt. John Murphy, chief of prosecution for the military commission. "It ends all appeals. This case is over."

Among other things, Khadr, as part of the agreement, waived his rights to appeal or to take action against the U.S.

In the plea deal, Khadr pleaded guilty Oct. 25 to killing Speer and four other charges, including attempted murder, conspiracy, providing material support to terrorists and spying.

According to a document listing facts in the case, Khadr killed Speer when he threw a grenade at him following a firefight between al-Qaeda operatives and U.S. forces in Afghanistan on July 27, 2002.

Khadr was 15 when he was taken into custody after taking part in a battle against U.S. soldiers in Afghanistan.

Canada has signed the UN Convention on the Rights of the Child, which protects children-turned-soldiers from prosecution.

Though more than 1,200 U.S. troops have been killed in Afghanistan, Khadr is the only detainee who has been charged and put on trial in connection with a death , Edney said.

With files from The Canadian Press