WikiLeaks, the whistle-blower website, is holding the documents in violation of US law, the US State Department says in a letter to site founder Julian Assange and his lawyer.
"We will not engage in a negotiation regarding the further release or dissemination of illegally obtained US government classified materials," State Department legal adviser Harold Koh wrote.
"As you know, if any of the materials you intend to publish were provided by any government officials, or any intermediary without proper authorisation, they were provided in violation of US law and without regard for the grave consequences of this action.
"As long as WikiLeaks holds such material, the violation of the law is ongoing."
US officials said Assange had sent a letter to the Department of State on Friday, in which he tried to address US concerns that WikiLeaks' planned release of classified documents placed individual persons at risk.
In his letter, Assange said he wanted information regarding individuals who might be "at significant risk of harm" because of WikiLeaks' actions, the officials said.
It was not immediately clear what other offers, if any, Assange's letter contained.
But Koh's response strongly condemned the anticipated release.
"Despite your stated desire to protect those lives, you have done the opposite and endangered the lives of countless individuals," the US legal adviser wrote.
"You have undermined your stated objective by disseminating this material widely, without redaction, and without regard to the security and sanctity of the lives your actions endanger."
Koh said the document release would have "grave consequences" and place at risk the lives of "countless innocent individuals", from journalists to human rights activists to bloggers and soldiers.
It would place at risk ongoing military operations, including operations to stop terrorists, and cooperation between countries, he added.
An independent French website reported that the leaks would be published simultaneously at 8.30am AEDT tomorrow by several Western newspapers.
The website, owni.fr, had previously launched an interface allowing the public to search the Iraq war logs published by WikiLeaks in October.
It said the New York Times, Britain's The Guardian, Germany's Der Spiegel, Spain's El Pais and France's Le Monde would release their first analysis of the documents late Sunday, with leaks expected to trickle out before then.