A US military judge in Guantanamo Bay has thrown out the US government's evidence against an Afghan detainee because it was obtained under coercion, a rights group said Friday.

The decision came late Wednesday in a preliminary hearing in the trial of Mohammed Jawad, arrested in Kabul in 2002 as a teenager on charges of throwing a grenade that wounded two American soldiers and their Afghan interpreter.

A representative for the American Civil Liberties Union (ACLU), who was present at the hearing at the tribunal at the US military base in Cuba, revealed that Jawad's confession had been rejected as evidence in the case.

"Colonel Stephen Henley held that evidence collected while Jawad was in US custody cannot be admitted in his trial" because the evidence was "gathered through coercive interrogations," the ACLU said in a press release.

Accused of having committed "war crimes," Jawad, who is now about 23 years old, is set to face trial on January 5 before a military commission at the Guantanamo prison.

"Previously, the government had told the judge that Jawad's alleged confessions were the centerpiece of its case against him," the ACLU said.

"If the government continues to prosecute this case, it will only provide further evidence that the military commissions system is a sham aimed at obtaining convictions regardless of the facts or the law," said ACLU staff attorney Hina Shamsi.

The lead prosecutor in Jawad's case, lieutenant Colonel Darrel Vandeveld, quit in September due to "ethical concerns."