US investigators probing leaks of defence secrets by whistleblowing website WikiLeaks tried to bribe a suspect to "infiltrate" its data and prevent further revealations, a media report said on Sunday.

Investigators interviewed Boston-area acquaintances of Bradley E. Manning, a military analyst charged with providing documents related to Afghanistan war to the website, in an effort to prevent additional leaks, The Washington Post reported citing a person questioned in the probe.

The investigators from the Army and the state department seemed to be "looking for classified documents that they thought to be in the Boston area", the acquaintance said on condition of anonymity. "I got the impression that we're still in the process of containing a leak."

The man, a computer expert who met Manning in January, said he told the investigators in mid-June that he knew of no such documents. The computer expert also said the army offered him cash to "infiltrate" WikiLeaks.

"I turned them down. I don't want anything to do with this cloak-and-dagger stuff," he was quoted as saying.

The interview was among at least two investigations conducted in the Boston area after Manning was accused of giving WikiLeaks state department cables and a video of a helicopter attack in which unarmed civilians were killed in Baghdad, The Post said.

Officials said they were investigating whether Manning leaked the Afghanistan documents made public last week, a disclosure that prompted condemnation from the Obama administration.

Army Criminal Investigation Division spokesman, Mr Chris Grey, however, declined to comment on the claim. "We've got an ongoing investigation," he said. "We don't discuss our techniques and tactics."

Another Manning acquaintance who was questioned said investigators "assumed that he was the one who did it and were trying to understand why, what was going on with him psychologically, to either make it so nobody gets to this point in the future or spot people who've gotten to this point and make sure they didn't do any damage."

This acquaintance, also a computer expert who spoke on the condition of anonymity, is affiliated with the Massachusetts Institute of Technology. He said he was interviewed twice in June in Cambridge, Massachusetts, shortly after Manning was detained. Manning was charged in July.

Meanwhile, friends and family are raising money for Manning's defence, including a private lawyer to augment the army-provided defence lawyer.

The San Francisco-based war resisters' group Courage to Resist has raised $11,418 and is aiming for $100,000, assuming a "sizable contribution from WikiLeaks", said Jeff Paterson, project director.

Manning has been transferred from Kuwait, where he had been detained, to Quantico. He was charged in military court in July and will have a preliminary hearing to determine if he should face a court martial.

White House officials are concerned that more potentially damaging information could be released by the group in the coming weeks.

A senior military official balked at a suggestion by Defence secretary, Mr Robert M. Gates, that the WikiLeaks disclosure could cause the Pentagon to limit the distribution of classified information to combat field units, where it is harder to monitor what analysts are downloading.

"Limiting intelligence to troops in combat is a non-starter," said the official. "It doesn't make sense to use WikiLeaks as a reason to limit information to the troops who need it." Such limits could "get soldiers killed", the official said.

The classified computer systems in Iraq and Afghanistan don't have the same safeguards that exist in the United States. "In the States, there are rack and scoring servers that watch where analysts go," the official said.