The WikiLeaks website appears close to releasing what the Pentagon fears is the largest cache of secret US documents in history - hundreds of thousands of intelligence reports compiled after the 2003 invasion of Iraq.
In a message posted to its Twitter page, the organisation said there was a "major WikiLeaks press conference in Europe coming up". WikiLeaks has not commented publicly on the imminent announcement.
The disclosure would be the most massive leak of secret documents in US history and defence officials are racing to contain the potential damage.
A team of more than 100 analysts from across the US military, led by the Defence Intelligence Agency, has been combing through the Iraq documents they think will be released.
Called the Information Review Task Force, its analysts have pored over the documents and used word searches to try to pull out names and other issues that would be particularly sensitive, officials said.
The taskforce has informed the US Central Command of some of the names of Iraqis and allies and of other information they believe might be released that could present a danger, officials said, noting that - unlike the WikiLeaks previous disclosure of some 77,000 documents from Afghanistan - in this case they had advance notice that names may be exposed.
That previous leak, in July, outraged the US military, which accused WikiLeaks of irresponsibility.
But the Associated Press news agency obtained a Pentagon letter reporting that no US intelligence sources or practices were compromised by the posting of secret Afghan war logs.
Although Pentagon officials still think the leaks could cause significant damage to US security interests, the assessment suggests some of the administration's worst fears about the July disclosure have so far failed to materialise.
Defence Secretary Robert Gates reported the conclusions in a letter on August 16 to Senator Carl Levin, chairman of the Senate Armed Services Committee, who had requested a Pentagon assessment.