A judge has ruled that the US government must prove that Bradley Manning knowingly helped al-Qaeda by leaking secret documents to the whistle-blower website WikiLeaks to convict him of aiding the enemy.

Judge Denise Lind's ruling at a preliminary hearing in a Baltimore, Maryland, court on Wednesday raises the bar for convicting Manning, a US army private, of the most serious charge he faces.

Manning, 25, has admitted leaking the documents but denies aiding the enemy.

Colonel Lind said the prosecution in the military tribunal must prove beyond a reasonable doubt that Manning had "reason to believe such information could be used to the injury of the US", by an armed group like al-Qaeda or another nation.

The judge also ruled that the government could call as a witness one of the commandos who took part in the 2011 raid that killed Osama bin Laden.

The Navy SEAL would testify on condition of anonymity that documents leaked to WikiLeaks were found in the al-Qaeda leader's compound in Abbottabad, Pakistan.

Prosecutors said the Navy SEAL collected digital evidence showing that the al-Qaeda leader requested and received from an associate some of the documents Manning had acknowledged sending to Julian Assange's WikiLeaks website.

Defence lawyers had argued that proof of receipt was irrelevant to whether Manning aided the enemy, the most serious charge he faces, punishable by life imprisonment.

Exposing 'bloodlust'

Manning is accused of sending hundreds of thousands of Iraq and Afghanistan battlefield reports, State Department diplomatic cables, other classified records and two battlefield video clips to WikiLeaks in 2009 and 2010 while working as an intelligence analyst in Baghdad.

He said in February that he had leaked secret war logs and diplomatic cables to WikiLeaks and he would plead guilty to 10 of the less serious charges against him, which could see him sentenced to 20 years in military custody.

Manning said in a statement he read aloud in court on February 28 that he sent the material to the anti-secrecy website to expose the American military's "bloodlust" and disregard for human life in Iraq and Afghanistan.

The hearing, due to end later on Wednesday, was the first since a group pressing for more government transparency flouted a military ban by releasing a secretly recorded audio clip of Manning's testimony.

The leak marked the first time since Manning was arrested in May 2010 that the world had heard his voice.

It was secretly taped on February 28 when Manning explained why he funnelled US military and State Department documents to WikiLeaks from November 2009 to May 2010.

As a result, mobile phones and recording devices, previously only banned inside the courtroom, are now outlawed in the press gallery as well, where the hearing is being broadcast.

Manning's trial is due to begin on June 3 at Fort Meade near Washington.