Lawyers for US soldier Bradley Manning mounted an aggressive defense at a hearing Friday on whether to court-martial him for spilling US diplomatic and military secrets to WikiLeaks.

Manning's lead attorney, David Coombs, went on the offensive shortly after proceedings got underway at this sprawling military base on Friday, accusing the presiding officer of bias and seeking his dismissal.

Lieutenant Colonel Paul Almanza, tasked with determining whether Manning should face a court-martial, rejected demands that he recuse himself, saying he could serve impartially despite working for the Justice Department in civilian life.

Coombs had questioned whether Almanza, an army reservist on leave from his job as a Justice Department attorney, should preside over the case while the department is conducting a probe of WikiLeaks and its founder Julian Assange.

A US Army legal expert told reporters the court could rule on Manning's appeal as early as Saturday, when the pre-trial hearing is set to reconvene at 10:00 am (1500 GMT). It could last up to a week.

Manning is suspected of downloading 260,000 US diplomatic cables, videos of US air strikes and US military reports from Afghanistan and Iraq while serving as a low-ranking intelligence analyst in Iraq and providing them to Assange, who has denied knowing the source of the material.

He faces life in prison if convicted of the most serious charge – aiding the enemy.

In seeking the officer's dismissal, Coombs said Almanza had also rejected most of the witnesses requested by the defense – a list that included President Barack Obama, Secretary of State Hillary Clinton and former defense secretary Robert Gates – and that was further evidence of bias.

"An individual looking at this from the outside, a reasonable person, would say clearly this is biased," Coombs said.

The defense lawyer also suggested that the documents that Manning is alleged to have leaked had done little damage to US national security. "Where's the harm?" he asked.

After consulting with his legal adviser, Almanza rejected the request he recuse himself, saying he was not involved in any Justice Department probe of WikiLeaks or Manning and that many of the witnesses sought by the defense were unnecessary to determine whether the US Army private should go to trial.

Coombs is appealing the decision to the Army Court of Criminal Appeals.

The back-and-forth over the recusal issue and four recesses for consultations consumed the entire day of proceedings and no witnesses were called.

Dressed in a green camouflage uniform of the 10th Mountain Division and wearing thick black glasses, Manning, who was arrested over 18 months ago, appeared composed throughout the day's proceedings.

He sat calmly at the defense table, fiddling with a pen, jotting down notes and leaning over occasionally to speak with Coombs and his two military-appointed defense attorneys.

Manning, who turns 24 on Saturday, spoke several times during the hearing, repeatedly answering ‘Yes, sir’ when asked by Almanza whether he understood the charges, was aware of his rights and was satisfied with his defense counsel.

The pre-trial hearing is being held in an austere courthouse at Fort Meade, headquarters of the top secret National Security Agency, and is being attended by dozens of members of the public and media from around the world.

A vigil was held by Manning supporters outside the gates. A witness said a man shouted "Bradley, you're a hero" inside the courtroom as the hearing wrapped up about 6.5 hours after it began.

In instant message chats with Adrian Lamo, the former computer hacker who turned him over to the US authorities, Manning expressed hope that the material he released would trigger ‘worldwide discussion, debates and reforms’.

"I want people to see the truth, regardless of who they are, because without information you cannot make informed decisions as a public," Manning said in the chat logs obtained and published by Wired.com.

Manley is the only suspect facing trial in the United States for the document dump – a massive intelligence breach that led to an embarrassing daily drip of diplomatic revelations for the United States and other governments.