WASHINGTON - The U.S. government is telling a federal appeals court that it has the authority to detain Canadian Omar Khadr, captured in Afghanistan when he was 15.

Khadr, now 21, is being held at the U.S. military prison in Guantanamo Bay, accused of killing a U.S. soldier with a grenade during a firefight in Afghanistan in 2002.

Attorneys for Khadr say international law bars governments from detaining people that young as enemy combatants and prosecuting them for war crimes.

The government, in a filing Friday with the U.S. Court of Appeals for the District of Columbia Circuit, says the military has the authority on the battlefield to capture and detain anyone, including juveniles, attacking and killing U.S. soldiers.

The court will hear arguments in the case on Sept. 4, in advance of Khadr's trial, slated for October.

Tapes of a February 2003 interrogation of the Toronto-born Khadr by the Canadian Security Intelligence Service were released Tuesday, showing Khadr shifting between a grinning teenager and a forlorn victim.