FORT MEADE, United States (AFP) - The US soldier accused of espionage for handing over classified files to WikiLeaks urged a military judge on Monday to dismiss several counts against him, including the serious charge that he "aided" Al-Qaeda by spilling secrets.

Bradley Manning's lawyers asked the court to dismiss charges he broke rules for using military computers, stole government "property," disclosed email addresses and aided the enemy when he gave classified files to the anti-secrecy website.

The move came as Manning's attorneys began to present their case, seeking to counter the prosecution's portrayal of the US Army private as an arrogant traitor who knew the leaked documents could fall into the hands of Al-Qaeda.

The defence opened the proceedings by playing a 39-minute video of a 2007 US helicopter attack in Baghdad that went viral in 2010 after Manning passed the footage to WikiLeaks, which released the clip under the title "Collateral Murder."

The disturbing video, from a cockpit gunsight, shows two Apache helicopters firing at a group of Iraqi men whom the crew mistakenly believed were carrying weapons.

Two of those killed in the assault were Iraqis working for the Reuters news agency.

Manning, 25, has admitted to giving WikiLeaks more than 700,000 secret military intelligence files and diplomatic cables in the worst leak of classified information in American history.

He has pleaded guilty to lesser offences that could carry a 20-year prison sentence.

But he is contesting 21 other charges, including the most serious count that he knew he was "aiding the enemy" by funneling the files to the website.

That charge carries a maximum penalty of life in prison.

The prosecution rested its case last week but suffered an embarrassing setback after acknowledging the military had lost the contract Manning signed laying out the terms of his access to classified information.

As the trial entered its sixth week at Fort Meade in Maryland, north of Washington, the first defence witness told the court Manning shined as one of the most talented members of an intelligence analysis unit, excelling at "data mining."

"He was our best analyst by far when it came to developing products," said Chief Warrant Officer Joshua Ehresman, who oversaw intelligence work produced by Manning and other enlisted soldiers.

Unlike other troops who often needed assignments to be spelled out in detail, Manning was the "go-to guy" and "would come up with exactly what you were looking for," he said.

Chief Warrant Officer Ehresman and two other witnesses said there were no rules in Manning's unit barring soldiers from running an executable file or program off of a CD.

Monday's testimony was meant to bolster the defense's argument that Manning did not exceed his authorised access to government databases or violate computer regulations for intelligence analysts.

While prosecutors say Manning exposed informants to danger by leaking military intelligence reports, Captain Steven Lim, who supervised Manning's section, testified that names of sources were not cited in the reports.

Intelligence sources in Iraq were "usually tracked by a number" with no name listed, Captain Lim told the court.

Among the witnesses due to appear this week is retired Colonel Morris Davis, a former prosecutor of terror suspects held at the "War on Terror" prison at Guantanamo Bay, Cuba.

Col Davis, who has since turned into a critic of how the US government prosecutes suspected militants, likely will be asked about documents on Guantanamo detainees that were part of Manning's document dump.

Defence lawyer David Coombs has described Manning as a "naive" but well-intentioned soldier, who wanted to shed light on US foreign policy abuses and the conduct of the wars in Iraq and Afghanistan.

Manning's case has taken on added importance in the aftermath of another round of embarrassing leaks from a former contractor for the National Security Agency.

Edward Snowden fled to Hong Kong and later to Moscow after handing over documents to the media revealing far-reaching US electronic surveillance of phone records and Internet traffic.