The Obama administration in the United States is warning online whistleblower WikiLeaks that its expected imminent release of classified State Department documents will put "countless" lives at risk, threaten global counterterrorism operations and jeopardize U.S. relations with its allies.

In a letter sent Saturday to WikiLeaks founder Julian Assange, the State Department's top lawyer says publication of the documents could do massive harm to the standing of the U.S. by exposing secret military operations and revealing dissidents' charges against foreign governments. The cables are expected to be released on Sunday.

State Department legal adviser Harold Koh sent the message in response to a letter from Assange and his lawyer in which they asked about the safety of people who might be named in the cables.

Koh said the U.S. will not co-operate or negotiate with WikiLeaks.