Omar Khadr, shown in this courtroom artist's sketch, attends a hearing at the U.S. naval base in Guantanamo Bay, Cuba, in January 2009. (Janet Hamlin, Pool/Associated Press)

An American military lawyer for Omar Khadr filed an emergency petition with the U.S. Supreme Court on Monday to try to halt the Canadian's upcoming trial at Guantanamo Bay, Cuba.

The trial is scheduled to begin Aug. 10.

Toronto-born Khadr, now 23, has been held at the military prison since he was captured in 2002, at the age of 15, during a firefight in Afghanistan with U.S. soldiers. He faces five charges in connection with the incident, the most serious being murder of a U.S. special forces soldier.

His attorney, army Lt.-Col. Jon Jackson, said he filed the petition with the top court because a U.S. federal appeals court in Washington had not acted on a request he filed four months ago.

Jackson argues that the offshore system for prosecuting war-crimes suspects is unconstitutional. Among other concerns, he said it is unfair because it is reserved only for non-U.S. citizens.

"The military commissions provide young Omar, a Canadian citizen, only second-class justice. This kind of discrimination is something we cannot stand for as a country," Jackson said.

Canadian appeal ongoing

The Canadian government is in the process of appealing a court ruling that ordered it to remedy its own breaches of Khadr's constitutional rights.

Federal Court Justice Russel Zinn ruled in July that the federal government had not met the standard set by the Supreme Court of Canada earlier in 2010 when it declared Khadr's rights had been violated by Canada at Guantanamo Bay.

The top court noted that CSIS officials obtained evidence from Khadr under "oppressive circumstances" during interrogations in Cuba in 2003, and then shared that evidence with U.S. officials.

Khadr's legal team in Canada has tried — through the courts — to force the government to demand Khadr be repatriated. He's the lone Western citizen remaining in detention at Guantanamo Bay.

One of his lawyers said the legal team wasn't surprised by the government's decision to appeal the latest court ruling, but said he doubted the Federal Court of Appeal could reach a decision before Khadr's scheduled Aug. 10 trial.

The Khadr case has been fraught with controversy since his capture, which came at an age when international conventions would require he be treated as a child soldier in need of rehabilitation and not a criminal. Also, various bits of the U.S. evidence against him have come into doubt since it was revealed that nobody saw Khadr throwing the grenade that killed the American special forces soldier.

With files from The Associated Press