A former child soldier has gone on trial at the prison camp in Guantanamo Bay in the US, the first detainee to face military justice under President Barack Obama.

Omar Khadr, a Canadian citizen, is accused of throwing a grenade that killed a US soldier during a battle in Afghanistan in 2002, when he was 15.

The UN Special Representative for Children, Radhika Coomaraswamy, said the trial might set a dangerous precedent for child soldiers worldwide.

Omar Khadr’s military lawyer Jon Jackson and military prosecutors will interview 15 officers before selecting at least five for the jury in the trial, which is expected to last at least three weeks.

In July, Mr Khadr threatened to boycott the process, which he labelled a ‘sham’.

He is the last remaining Westerner at Guantanamo and the only detainee charged with murder.

He is alleged to have been trained by al-Qaeda and joined a bomb-making network organised by Osama bin Laden.

Canada has not requested Mr Khadr’s return, preferring to allow the US trial to run its course.

Mr Khadr, who was seriously wounded in Afghanistan and lost vision in his left eye, has so far refused Washington's offer of 30 years in prison - including 25 in Canada - in exchange for a guilty plea.