WikiLeaks suspect Bradley Manning declined to enter a plea as the US Army private was formally charged on Thursday with turning over a cache of classified US documents to WikiLeaks.

Manning, a 24-year-old former intelligence analyst, was charged with 22 counts, the most serious of which is "aiding the enemy," which could send him to prison for life.

Dressed in a US Army uniform and flanked by his lawyers, Manning was silent during the 45-minute arraignment, responding only "Yes, your honor" when asked by military judge Denise Lind whether he understood the charges against him.

Manning deferred entering a plea, which he is not required to do until the start of the court-martial. He also declined to say whether he preferred to be tried before a single military judge or a military jury.

During the hearing, the judge asked attorneys for the prosecution and the defense when they wanted the court-martial proceedings to begin.

Army prosecutors opted for an August 3 start date but Manning's civilian attorney, David Coombs, asked for the court-martial to begin in June.

"As of today, Bradley Manning has been in confinement for 635 days," Coombs said.

Manning is accused of passing hundreds of thousands of military field reports from Iraq and Afghanistan and US diplomatic cables to WikiLeaks between November 2009 and May 2010, when he was serving in Iraq.

The leak of the military documents shed light on civilian deaths, while the diplomatic cables sparked a firestorm by disclosing the private remarks of heads of state and candid observations by senior US officials.

The US government slammed the disclosure of the documents by WikiLeaks, saying it threatened national security and the lives of foreigners working with the military and US embassies.

WikiLeaks supporters view the site as a whistleblower that exposed US wrongdoing and see Manning as a political prisoner.

Army investigators told a hearing in December that contact information for WikiLeaks founder Julian Assange, military reports, cables and other classified material had been found on computers and storage devices used by Manning.

Manning's defense attorneys have portrayed him as suffering from "gender identity disorder" and said he had created an online female alter ego called "Breanna Manning."

The Bradley Manning Support Network said this month that Manning had been nominated for the Nobel Peace Prize by members of the Icelandic parliament.

It is not possible to verify who has been nominated for the awards.

Assange has meanwhile been in Britain fighting extradition to Sweden, where he faces sexual assault accusations.

He has denied the allegations, saying they are politically motivated.