The US military says the expected release of classified documents on the Wikileaks website could endanger US and allied troops and Iraqi civilians.
The whistleblower website is thought to be about to post hundreds of thousands of US military files on the Iraq war.
Nato chief Anders Fogh Rasmussen said the leak could have "very negative security implications".
A US defence spokesman said the documents concerned "significant activities" reported by units.
The Pentagon has assembled a team of more than 100 analysts to prepare for the release of the documents, which was expected later on Friday.
A US defence department spokesman said the "stolen" documents should be returned to avoid potentially damaging information being released.
"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," said spokesman Geoff Morrell.
He said the documents were "essentially snapshots of events, both tragic and mundane, and do not tell the whole story.
"That said, the period covered by these reports has been well-chronicled in news stories, books and films and the release of these field reports does not bring new understanding to Iraq's past."
He added: "However, it does expose secret information that could make our troops even more vulnerable to attack in the future."
The Wikileaks website is currently down for "scheduled maintenance", but promised on its Twitter feed a "major announcement in Europe" on Saturday morning.
Speaking earlier on Friday, Nato Secretary General Anders Fogh Rasmussen said the "leaks may put soldiers as well as civilians at risk".
Wikileaks has already angered the US government and military with its release in July of more than 70,000 classified documents on the war in Afghanistan.
Military officials warned at the time the leak could lead to the deaths of US soldiers and Afghan civilians because some of the documents contained the names of locals who had helped coalition forces.
But US Secretary of Defence Robert Gates said in a letter to the head of the Senate Armed Services Committee that the leak had not revealed any "sensitive intelligence sources or methods".
There have been fears such leaks could damage US intelligence sharing with other nations, as well as intelligence sharing between US agencies.
The investigation into the Afghan leak has focused on Bradley Manning, a US army intelligence analyst who is in custody and has been charged with providing Wikileaks with a classified video of a US helicopter attack in Iraq in 2007 in which a dozen people were killed.