Image caption Omar Khadr's defence say he was forced into war

A former child combatant has gone on trial at Guantanamo Bay, the first detainee to face military justice under President Barack Obama.

Canadian citizen Omar Khadr, now 23, is accused of throwing a grenade that killed a US soldier during a gun battle in Afghanistan in 2002, when he was 15.

He alleges that he was tortured into confessing to the murder.

A UN envoy, Radhika Coomaraswamy, said the trial would set a dangerous precedent for child soldiers worldwide.

However, the judge said the prosecution must show that Mr Khadr had had intent to commit a crime, and he told jurors they could consider his age in making their decision.

Mr Obama had promised to close the controversial detention facility by January this year.

Mr Khadr's trial will shine further light on both the camp and Mr Obama's failure to close it, the BBC's Kim Ghattas reports from Washington.

'Clear standards'

Military officers in the jury pool indicated that they saw no problem with trying Mr Khadr.

"Does anyone believe that juveniles should not be prosecuted for violent offences?" Prosecutor Jeff Groharing asked them.

"Does anyone feel the accused should be held to a different standard because he was 15 years old at the time of the alleged offences?"

None said they held those beliefs.

The judge, Col Patrick Parrish, said the jury could consider the age of the defendant - now a tall, broad-shouldered and bushy-bearded man - at the time of his alleged crime.

"It's certainly something you may consider in deciding whether the government has proved its case beyond a reasonable doubt," he said.

Opening arguments are due to begin on Wednesday in a trial expected to last about three weeks.

Ms Coomaraswamy, special envoy for children in armed conflict, said no child had been prosecuted for a war crime since World War II.

"Juvenile justice standards are clear: children should not be tried before military tribunals," she said.

'Sad, pathetic chapter'

Prosecutors describe Mr Khadr as an al-Qaeda militant, who killed Sgt Christopher Speer when a US special forces unit raided one of the group's compounds in eastern Afghanistan.

Image caption Sgt Speer was 28 when he was killed in Afghanistan in 2002

Mr Khadr was shot twice in the back during the firefight and was flown to Guantanamo shortly after.

But the defence say Mr Khadr was himself a victim, forced into war by a family with close ties to al-Qaeda leader Osama Bin Laden.

The US says his father, Ahmed Said Khadr, was an al-Qaeda financier before he was killed in a gun battle in Pakistan in 2003.

The judge ruled earlier that the confessions Mr Khadr allegedly made while in custody could feature in the trial.

Lt Col Jon Jackson, lawyer for the accused, had argued that they were obtained through "degrading treatment", including indirect threats of rape and death.

On Tuesday, Lt Col Jackson attacked Mr Obama for allowing both Guantanamo to remain open and the tribunal to go ahead.

"President Obama has decided to write the next sad, pathetic chapter in the book of military commissions," he said.

"So forever Obama's military commissions will be remembered for starting with a case against a child soldier."

The charges include murder, conspiracy and support of terrorism. Mr Khadr has refused an offer of 30 years in jail in exchange for a guilty plea. A conviction carries a maximum sentence of life in prison.

Canada has declined to intervene in Mr Khadr's trial, despite federal court rulings in Ottawa that his rights were violated when Canadian agents interrogated him at Guantanamo Bay.