FORT MEADE, Md. -- Lawyers for a U.S. Army soldier who gave thousands of classified documents to the secrets-spilling website WikiLeaks opened their defence at his trial Monday with leaked video of a helicopter attack in Baghdad -- footage that portrayed the military in a negative light.

The video is the basis of an espionage charge alleging Bradley Manning had unauthorized possession of U.S. defence information.

Manning has admitted to leaking the cockpit video showing a 2007 attack that killed 11 men, including a Reuters news photographer and his driver. The aircrew can be heard laughing and calling the victims "dead bastards." Manning has said he was troubled by their behaviour and by the U.S. military's refusal to release it.

The Pentagon concluded that the troops reasonably mistook the journalists for enemy combatants. WikiLeaks posted it in April 2010 under the title "Collateral Murder."

The 25-year-old Manning admitted he leaked the video and hundreds of thousands of classified Iraq and Afghanistan war logs and diplomatic cables while working as an intelligence analyst in Baghdad in 2009 and 2010. Manning says he leaked the material because he was troubled by what it revealed about U.S. foreign policy.

Manning's defence team asked the military judge to acquit him of seven charges for lack of incriminating evidence, including the most serious charge of aiding the enemy, which carries a possible life sentence. The government has until Thursday to respond, the judge said.

Manning faces 21 contested counts. He pleaded guilty in February to reduced versions of some charges. He faces up to 20 years for the admitted offences.

Prospective witnesses include Harvard law professor Yochai Benkler, who has written that leaking something to WikiLeaks is no different than leaking it to The New York Times. Benkler's testimony could refute the government's assertion that Manning knowingly gave intelligence to the enemy because he knew al-Qaida members would see what WikiLeaks posted online.

On Monday, the defence called its first witness, Chief Warrant Officer 2 Joshua Ehresman, to establish the wide authorized access Manning and other intelligence analysts had. Ehresman testified that Manning and other intelligence analysts scoured a classified computer network for bits of information needed by field commanders.

Ehresman said the job meant "pulling everything you can from all intelligence assets." Manning was "the go-to guy" among the analysts in his unit in Iraq and the most productive worker, Ehresman said.

Defence witness Capt. Steven Lim, who helped supervise Manning's unit in Iraq, testified he gave the analysts the Internet address for State Department diplomatic cables and told them to incorporate the classified documents into their work. But Lim said he never told the analysts to look at cables involving countries other than Iraq.

The defence also called Lauren McNamar, a transgender woman with whom Manning, who is gay, had a series of online chats from February to August 2009. McNamara, then Zachary Antolak, verified chat log excerpts in which Manning discussed his military service.

"military is all f'd up... contracts with closed source developers with incompatible software... drives me NUTS," Manning wrote on Feb. 21, 2009.