US NAVAL BASE AT GUANTANAMO BAY - A US MILITARY tribunal sentenced former child soldier Omar Khadr to 40 years in prison on Sunday, but a plea deal means the Canadian citizen will serve up to eight years behind bars.

A seven-member military panel deliberated for nearly nine hours over a two-day period before reaching their decision for Khadr, who pleaded guilty earlier this month to throwing a grenade that killed a US sergeant in Afghanistan in 2002, when he was just 15.

Judge Patrick Parrish, a US Army colonel, said that under a plea agreement with US authorities to avoid a life sentence, Khadr would serve one year at Guantanamo Bay and the rest in Canada, pending Ottawa's approval. Khadr, now 24, became the third Guantanamo detainee to plead guilty and the fifth to face court proceedings before military commissions, George W. Bush-era war tribunals reformed and reinstated by President Barack Obama.

He is the last Westerner held at Guantanamo Bay, the US naval base where 174 'war on terror' detainees remain. 'The world is watching,' prosecuting attorney Jeffrey Groharing told Khadr on Saturday in closing arguments at a sentencing hearing at Guantanamo.

'Your sentence will send a message to Al-Qaeda and others whose aims and goals are to kill and cause chaos around the world.' He called Khadr 'an accomplished terrorist (who) committed adult offences' and requested no fewer than 25 additional years in prison.

Khadr, who has already spent eight years at the Guantanamo prison camp, admitted in his plea agreement to throwing the grenade that killed sergeant Christopher Speer in Afghanistan in July 2002 and told his widow that he was sorry. He pleaded guilty to murder in violation of the laws of war, providing material assistance to a terrorist organisation and espionage.

Ex-child soldier jailed 8 years