U.S. officials said Friday they were racing to contain the damage from the imminent release, while NATO's top official told reporters he feared that lives could be put at risk by the mammoth disclosure.
NATO chief Anders Fogh Rasmussen said any release would create "a very unfortunate situation."
"I can't comment on the details of the exact impact on security, but in general I can tell you that such leaks ... may have a very negative security impact for people involved," he told reporters Friday in Berlin following a meeting with German Chancellor Angela Merkel.
The WikiLeaks documents could amount to a classified history of the war in Iraq. In a posting to Twitter, the secret-spilling website said there would be a "major WikiLeaks announcement in Europe at 10 a.m." (0900 GMT, 5.a.m. EDT) Saturday. The group has revealed almost nothing publicly about the nature of the announcement.
Meanwhile, a team of more than a hundred analysts from across the U.S. military, lead by the Defense Intelligence Agency, has been combing through the Iraq documents they think will be released in anticipation of the leak.
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 have said.
The task force has informed U.S. Central Command of some of the names of Iraqis and allies and other information they believe might be released that could present a danger, officials have 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.
Once officials see what is publicly released, the command "can quickly push the information down" to forces in Iraq, Marine Corps Col. Dave Lapan, a Defense Department spokesman, said Friday in Washington.
"Centcom can jump into action and take whatever mitigating steps" might be needed, Lapan said.
The documents could shed light on the root causes of the insurgency, for instance, or the growth of sectarian violence that blighted Baghdad and other Iraqi cities. It may also give a behind-the-scenes glimpse at some of the major episodes of the war - like the manhunt for insurgent chief Abu Musab al-Zarqawi, or the killing of U.S. security contractors on March 31, 2004, by a mob in Fallujah, an incident which ultimately led to the U.S. assault on the Iraqi city.
Wikileaks' previous release in July of secret war documents from Iraq and Afghanistan outraged the Pentagon, which accused the group of being irresponsible. Fogh Rasmussen said Friday that leaks of this nature "may put soldiers as well as civilians at risk."
It appears that those fears - which the military has invoked in its appeal to WikiLeaks and the media not to publish the documents - have yet to materialize. A Pentagon letter obtained by The Associated Press reported that no U.S. intelligence sources or practices were compromised by the Afghan war logs' disclosure.
Still, the military feels any classified documents release can harm national security and raise fears for people who might consider cooperating with the U.S. in the future, Lapan said.
Ryan Crocker, U.S. ambassador to Iraq from 2007-08, said the disclosures would be more worrisome if the U.S. were still fully engaged in combat in Iraq - but he still sees it as a major problem.
"I'd really be worried if - as looks to be the case - you have Iraqi political figures named in a context or a connection that can make them politically and physically vulnerable to their adversaries," he told a conference Friday at the Center for Strategic and International Studies in Washington.
"That has an utterly chilling effect on the willingness of political figures to talk to us - not just in Iraq but anywhere in the world," he said.
Jelinek reported from Washington. National Security Writer Robert Burns in Washington and Associated Press Writer Melissa Eddy in Berlin contributed to this report.