U.S. officials condemned the move and 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.

In a posting to Twitter, the secret-spilling website said there would be a "major WikiLeaks announcement in Europe" at 0900 GMT (5 a.m. EDT) Saturday. The group has revealed almost nothing publicly about the nature of the announcement.

A U.S. Defense Department spokesman, Marine Corps Col. Dave Lapan, echoed Rasmussen's stance, urging WikiLeaks to return the stolen material — some 400,000 secret files on Iraq that Pentagon officials believe someone slipped to the organization.

"We deplore WikiLeaks for inducing individuals to break the law, leak classified documents and then cavalierly share that secret information with the world, including our enemies," Lapan said. "By disclosing such sensitive information, WikiLeaks continues to put at risk the lives of our troops, their coalition partners and those Iraqis and Afghans working with us."