The US government has accused Bradley Manning of dumping a massive and indiscriminate trove of state secrets "into the lap of the enemy" by transmitting classified documents to the open information website WikiLeaks.

In an hour-long opening statement for the prosecution on Monday, Captain Joe Morrow told the court martial that the US army private had been motivated by a craving for "notoriety" that had led him to disregard his extensive training and to the "aid of our adversaries".

The prosecution statement made new allegations about the links between Manning and Julian Assange, founder of WikiLeaks. Morrow said that Manning had directly assisted in the editing of the Apache helicopter video of a US attack on civilians in Baghdad, and the court was shown extracts of a chat log between the soldier and Assange.

Prosecutors also alleged that Manning had been guided in his searches by the WikiLeaks "most wanted list".

Manning has previously said that he corresponded online with someone he believed to be Assange but never confirmed the person's identity. WikiLeaks has been careful never to confirm or deny Manning was the source of the documents.

Assange has been holed up in the Ecuadorean embassy in London to avoid extradition to Sweden on allegations of sex crimes.

Manning sat calmly in the courtroom in his dark green dress uniform as the trial began. He chose to have his case heard by a judge instead of a jury.

The judge, Colonel Denise Lind, said last month she would close parts of the trial to the public to protect classified material.

Lind began the trial by asking Manning a number of procedural questions, including whether he was willing to have the case decided by a judge rather than a jury and whether he was satisfied with his defence team. "Yes, your honour," Manning replied.

Of the 21 counts faced by the army private on Monday, by far the most serious is that he knowingly gave intelligence information to al-Qaida by transmitting hundreds of thousands of classified documents to WikiLeaks.

Manning is accused of "aiding the enemy", in violation of Article 104 of the Uniform Code of Military Justice. The prosecution alleges that by indirectly unleashing a torrent of secrets onto the internet, Mannning in effect made it available to Osama bin Laden and his cohorts.

The solider claims he released the material to WikiLeaks to expose the American military's "bloodlust" and disregard for human life in Iraq and Afghanistan. He says he did not believe the information would harm the US.

Manning has already pleaded guilty to lesser offences, that he transmitted classified information to WikiLeaks, carrying a possible maximum sentence of 20 years.

Between November 2009 and May 2010 Manning downloaded massive files, stored in secure US intelligence databases, from his computer at an army operating base in Iraq, where he was working as an intelligence analyst. He then transmitted the files to an encrypted whistleblower channel set up by WikiLeaks. But prosecutors did not accept the pleas and the case proceeded to court martial.

In the course of pre-trial hearings, military prosecutors have outlined the basic skeleton of their case against Manning. They have indicated that they would seek to show that Osama bin Laden personally instructed an aide to download elements of WikiLeaks, including the Afghan war logs, on to digital storage devices so that he could read them.

Manning's trial, which is slated to last three months, opens against a backdrop of mounting unease about the increasingly aggressive stance the US government is taking against official leakers. The Obama administration has launched six prosecutions under the Espionage Act, twice as many as all previous presidencies combined, of which only Manning's has gone to trial.

The Department of Justice is already under fire for its controversial secret seizures of phone records of Associated Press reporters and of a Fox News reporter, James Rosen, investigating North Korean nuclear tests.

Associated Press in Fort Meade contributed to this report