US Secretary of State Hillary Clinton recalled Thursday "the damaging action" caused by the mass release of classified US documents, one day before a US soldier was to be tried in the case.

"In an age where so much information is flying through cyberspace, we all have to be aware of the fact that some information which is sensitive, which does affect the security of individuals and relationships deserves to be protected," Clinton said as a military tribunal near Washington prepared to begin its prosecution of Bradley Manning.

Manning, a private first class in the US Army, was arrested on May 26, 2010 in the case. He is accused of providing hundreds of thousands of such documents to WikiLeaks, which in turn released them publicly.

The massive document dump "was a very unfortunate and damaging action... that put at risk individuals and relationships to an extent that we took very seriously and launched a very vigorous diplomatic effort to try to counter," Clinton said.

Manning, who turns 24 on Saturday, is scheduled to appear at a preliminary military hearing at Fort Meade near Baltimore, Maryland, headquarters of the top secret National Security Agency.

Manning is said to have furnished WikiLeaks with US diplomatic cables, videos and military reports from Afghanistan and Iraq while serving as an intelligence analyst on a military base near Baghdad between November 2009 and May 2010.