The confessions of Omar Khadr, a Canadian citizen charged with terrorism, can be used as evidence in his trial, even though they may have been obtained through torture, a US military judge has ruled.
Lawyers for Khadr claimed statements to military interrogators were illegally obtained through torture and asked a US war crimes court to throw them out.
That request was denied on Monday by a military judge at the US naval base at Guantanamo Bay, Cuba.
US forces captured Khadr in Afghanistan in July 2002, when he was just 15 years old.
"Omar Khadr could potentially be the first child soldier to be prosecuted for war crimes in modern history," Al Jazeera's Monica Villamizar said, reporting from Guantanamo Bay.
"Under international law, children captured in war should be treated as victims and not perpetrators."
Khadr is accused of killing a US soldier after throwing a grenade at the end of a four-hour US bombardment of an al-Qaeda compound in the eastern Afghan city of Khost.
The Canadian citizen, who is now 23, has refused a plea deal.
He faces a maximum life sentence if convicted of charges that include conspiring to commit terrorism and murder.
Allegations of abuse
Khadr's lawyers deny that he threw the grenade and contend that the prosecution is relying on confessions extracted following abuse.
Khadr's case is the first to go to trial under the system of military commissions for detainees captured by US forces in a global campaign following the September 11, 2001, attacks on the US.
Obama had sought to close the detention centre that has been the object of international condemnation, but he has faced congressional opposition on transferring the detainees to US soil.
The president has introduced some changes designed to extend more legal protections to detainees, but the tribunals' long-term future remains uncertain.
But Navy Captain David Iglesias, a lawyer and spokesman for the military commission's prosecutors at Guantanamo Bay, told Al Jazeera that the tribunals have improved and that Khadr can get a fair trial.
"It is a case that has been pending for many, many years. The government is ready to go forward. To what extent it has international repercussions is beyond anybody's guess," he said.
"The Military Commissions Act has been revised. It is a much better law than what it was under the 2006 Act.
"I believe based on my experience it is a fair system."
Our correspondent said that Khadr's attitude has changed drastically since his first court appearance in 2006.
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"He was very co-operative with the military system [but] now it is completely different," she said.
"Khadr's lawyer told me recently that he is trying to convince [Khadr] to be co-operative: to show up in trial and not [to] fire his only military defence lawyer who was assigned to him.
"Khadr has expressed in the past that he want[s] to be convicted, [to] show the world how unfair this system is ... and to show that the US will eventually convict child soldiers."
In a letter to Dennis Edney, his Canadian lawyer, published in newspapers in Canada and the US, Khadr said the trial may show the world how unfair the process is.
"The world doesn't get it, so it might work if the world sees the US sentencing a child to life in prison, it might show the world how unfair and sham this process is," Khadr said.
"And if the world doesn't see all this, to what world am I being released to? A world of hate ... and discrimination."
Separately this week, the Pentagon is also preparing to hold a military commission for the sentencing of Ibrahim Ahmed Mahmoud al-Qosi, a Sudanese detainee at Guantanamo.
Al-Qosi is accused of acting as accountant and aide to Osama bin Laden, the al-Qaeda chief, in the 1990s when the network was centred in Sudan and Afghanistan.
He is also accused of later working as bin Laden's bodyguard.
Al-Qosi pleaded guilty last month to one count each of conspiracy and providing material support to terrorism as part of a plea deal with prosecutors.
The 50-year-old had faced a potential life sentence if convicted at trial.
On Monday, a US military judge ordered that the plea deal, which put a cap on al-Qosi's sentence, be sealed.
The judge, Air Force Lieutenant Colonel Nancy Paul, said the deal limiting how much more time detainee al-Qosi spends in confinement will not be revealed until after his release.
She said that condition of the plea bargain was requested by the government and agreed to by the detainee's lawyers.
A jury of military officers is expected to begin deliberating al-Qosi's sentence on Tuesday, but officials overseeing the tribunals will reject their decision if it exceeds the terms of the plea bargain.
A longer sentence could be applied, however, if al-Qosi did something to break the terms of the plea agreement.
Since 2001, four men have been convicted of terrorism-related charges in Guantanamo military trials, two of whom pleaded guilty, while US federal courts have sentenced about 200 other suspects over the same period.