The US government must prove that army private Bradley Manning knowingly helped Al-Qaeda and other American foes by handing over secret documents to WikiLeaks, a judge ruled Wednesday.

The order by Judge Denise Lind at a preliminary hearing raises the bar for convicting Manning of "aiding the enemy," the most serious charge he faces.

Lind also ruled that the government can 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 anonymously at an "alternate secure location," and state that documents given to WikiLeaks were found in the late Al-Qaeda leader's compound in the Pakistani garrison town of Abbottabad.

The commando will be wearing "light disguise," Lind said.

Three other secret witnesses will also be heard in closed session, while another 24 government witnesses -- including ambassadors and officials from the Pentagon, State Department and intelligence services.

Manning admitted in February to leaking a trove of confidential war logs and diplomatic cables to Julian Assange's anti-secrecy website and said he would plead guilty to 10 of the less serious charges against him, which could see him sentenced to 20 years in military custody.

The 25-year-old, who was working as a US Army intelligence analyst when he was arrested in Iraq in 2010, has denied aiding the enemy, which would carry a life sentence.

Lind said US prosecutors 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 another nation or an armed group like Al-Qaeda.

The judge thus discarded a defense motion to "preclude evidence that Al-Qaeda or Al-Qaeda in the Arabian Peninsula allegedly received" the information in question.

She also made public for the first time a written version of two of her three rulings of the day, after media groups and rights activists condemned the lack of information being shared in the proceedings.

The hearing 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 has heard his voice.

It was secretly taped on February 28 when the accused explained why he funneled a trove of US military and State Department documents to WikiLeaks, between November 2009 and May 2010.

"To say that the judge was unhappy about this violation of the rules of the court would be an understatement," a military spokeswoman told reporters covering the hearing.

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.

"This media operation center is a privilege, not a requirement. Privileges can be taken away," the spokeswoman said.

Manning is set to go before a full court martial on June 3, with the trial expected to last 12 weeks at Fort Meade, near Washington. Preliminary hearings are also scheduled to take place behind closed doors on May 7 and 8.