By David Ljunggren

OTTAWA, June 14 (Reuters) - Canada must press the United States to release a young Canadian man who has been imprisoned at Guantanamo Bay for five years, Amnesty International and five former foreign ministers said on Thursday.

Last week a U.S. military judge dropped charges against Omar Khadr, who was captured in Afghanistan at the age of 15. The Pentagon said last week it would appeal that ruling.

Khadr, now 20, was accused of killing a U.S. soldier at a suspected al Qaeda compound in Afghanistan in 2002.

Alex Neve, head of Amnesty's Canadian chapter, said it was clear that Khadr and the 380 other prisoners held at the U.S. naval base on Cuba had no chance of being treated fairly.

"It is time, long past time in fact, for the Canadian government to intervene publicly and forcefully and unequivocally demand that Omar Khadr's stay at Guantanamo Bay must now come to an end," he told a news conference.

"His case should be dealt with appropriately under the Canadian justice system, taking account of the evidence against him as well as the fact that Omar Khadr was a 15-year-old minor in a war zone ... Canada can no longer remain silent."

Foreign Minister Peter MacKay said on Wednesday that he would not approach Washington about the case "until such time as the process and the appeals process has been exhausted".

Whether Ottawa would want Khadr back is another matter, since the Conservative government supports the U.S.-led war on terror and has promised to improve ties with Washington.

Neve said Britain, France, Germany and Australia had succeeded in repatriating their citizens from the base.

"This truly is a deep, unforgivable injustice," said Neve, who released an open letter to Prime Minister Stephen Harper urging Ottawa to act. The letter was signed by five former Canadian foreign ministers as well as 111 academics, lawyers and legislators.

MacKay said he had asked U.S. Secretary of State Condoleezza Rice to ensure that a medical and psychological assessment be done on Khadr.

Neve said Ottawa's reluctance to act could be linked to the family's notoriety. Khadr's father Ahmed Said Khadr, an alleged al Qaeda financier and close friend of Osama bin Laden, was killed in a battle with Pakistani forces in 2003.

Omar's brother Abdullah Khadr is facing extradition to the United States on charges of gun-running and conspiracy to murder Americans abroad.

A middle brother, Abdurahman Khadr, was also a prisoner at Guantanamo, but he was later freed and told Canadian media he had been asked to work for the CIA.

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