The State Department's letter to Assange, released late Saturday night, comes as U.S. officials have in recent days warned foreign governments that the documents could expose sensitive information and harm relations with the United States. The documents are expected to be released this weekend.

The letter was apparently written in response to a request by Assange for information on any individuals who may be "at significant risk of harm" by the disclosure of the documents. State DepLegal Adviser Harold Hongju Koh wrote that the Obama administration will "not engage in a negotiation regarding the further release or dissemination of illegally obtained U.S. Government classified materials."

Their release, he wrote, would place at risk ongoing military operations, including those aimed at stopping terrorists and human traffickers, and could harm relations among countries that work together "to confront common challenges from terrorism to pandemic diseases to nuclear proliferation that threaten global stability."

Koh urged Assange to not publish the materials and destroy all copies.

According to the letter, the State Department has been in contact with representatives of The New York Times, The Guardian newspaper of Britain and Der Spiegel of Germany, each of which has already received the WikiLeaks documents.

The State Department has prepared for the possible release by reviewing thousands of diplomatic cables and "assessing the potential consequences of the public release of these documents," spokesmann P.J. Crowley said last week.

Crowley said the consequences to American interests could be severe. The cables, for instance, could reveal that senior government officials in other countries are the sources of embarrassing information about the inner workings of those governments, thus making it more difficult for the State Department to obtain such intelligence in the future.

WikiLeaks has bedeviled the Obama administration with a series of damaging revelations about U.S. policy overseas. In July, it released more than 70,000 military reports on the war in Afghanistan and in October nearly 400,000 reports on the Iraq war.

WikiLeaks has not disclosed the source of the materials. But suspicion has centered on Pfc. Bradley Manning, 22, an Army intelligence analyst whom the military arrested this year, charging him with the downloading and transfer of classified material.