TORONTO, May 8 (Reuters) - The Canadian government said on Friday it would appeal a court order that it press Washington to release Omar Khadr, a Canadian held in the U.S. military prison at Guantanamo Bay, Cuba.

The ruling last month by Canada's Federal Court was a major embarrassment for Prime Minister Stephen Harper. He has resolutely brushed off demands that he intercede on behalf of Khadr, who has been held in the prison since October 2002.

"The government of Canada has consistently stated that Omar Khadr faces serious charges ... after careful consideration of the legal merits of the ruling from the Canadian Federal Court issued on April 23, the Government of Canada has decided to appeal," Alain Cacchione, a spokesman for Canada's Foreign Affairs Department, said in an e-mail.

Khadr is charged with killing a U.S. soldier during a firefight in Afghanistan in July 2002, when he was 15. Khadr, now 22, is the only citizen of a Western country still imprisoned at Guantanamo.

Opposition legislators and other critics have long argued that Canada should press for the release of Khadr, who they say was a child soldier when the suspected killing took place.

Harper and his Conservative government insist Khadr was charged with a very serious crime and that the proceedings against him should be allowed to unfold.

The April court ruling said "the ongoing refusal of Canada to request Mr. Khadr's repatriation to Canada offends a principle of fundamental justice" because Ottawa is obliged to ensure Khadr is being treated properly.

Last June, Canada's Supreme Court said Canadian secret agents may have been complicit in torture by interrogating Khadr in 2003 when they knew he had likely been abused by U.S. military investigators.

Although Canadian-born, Khadr is from a family that lived much of the time in Pakistan and had close ties to al Qaeda -- at one time staying in Osama bin Laden's compound in Afghanistan.

The U.S. case against Khadr was halted in January to give President Barack Obama time to decide whether to scrap the Guantanamo war crimes tribunals.

Obama has promised to shut down the prison camp at the U.S. military base, which was opened by the Bush administration in 2002 to hold and interrogate suspected al Qaeda and Taliban members after the Sept. 11 attacks. Critics have condemned the facility as a symbol of abuses in Washington's war on terrorism. (Reporting by Jeffrey Hodgson; Editing by Peter Cooney)

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