The Obama administration has told whistleblower WikiLeaks that its expected imminent release of classified State Department cables will put "countless" lives at risk, threaten global counterterrorism operations and jeopardize U.S. relations with its allies.

In a highly unusual step reflecting the administration's grave concerns about the ramifications of the move, the State Department late Saturday released a letter from its top lawyer to WikiLeaks founder Julian Assange and his attorney telling them that publication of the documents would be illegal and demanding that they stop it.

It also said the U.S. government would not cooperate with WikiLeaks in trying to scrub the cables of information that might put sources and methods of intelligence gathering and diplomatic reporting at risk.

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The letter from State Department legal adviser Harold Koh was released as U.S. diplomats around the world are scrambling to warn foreign governments about what might be in the secret documents that are believed to contain highly sensitive assessments about world leaders, their policies and America's attempts to lobby them.

In the letter, Koh said the publication of some 250,000 secret diplomatic cables by WikiLeaks, which is expected on Sunday, will "place at risk the lives of countless innocent individuals," ''place at risk on-going military operations," and "place at risk on-going cooperation between countries."

'Grave consequences'

"They were provided in violation of U.S. law and without regard for the grave consequences of this action," he said. Koh said WikiLeaks should not publish the documents, return them to the U.S. government and destroy any copies it may have in its possession or in computer databases.

The State Department said Koh's message was a response to a letter received on Friday by the U.S. ambassador to Britain, Louis Susman, from Assange and his lawyer, Jennifer Robinson. The department said that letter asked for information "regarding individuals who may be 'at significant risk of harm' because of" the release of the documents.

"Despite your stated desire to protect those lives, you have done the opposite and endangered the lives of countless individuals," Koh wrote in reply. "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."

He said the U.S government would not deal with WikiLeaks at all in determining what may or may not released.

"We will not engage in a negotiation regarding the further release or dissemination of illegally obtained U.S. government classified materials," wrote Koh, who is considered to be one of the world's top experts in international law and was reportedly considered for a seat on the Supreme Court.

WikiLeaks is expected to post the documents online on Sunday and Koh said the U.S. government had been told that The New York Times, the British newspaper the Guardian and the German news magazine Der Spiegel had prior access to them.

The release of Koh's letter comes as Secretary of State Hillary Rodham Clinton and other top U.S. officials are reaching out to numerous countries about the expected WikiLeaks release.

Clinton spoke to leaders in China, Germany, Saudi Arabia, the United Arab Emirates, Britain, France and Afghanistan on Friday, according to State Department spokesman P.J. Crowley. Canada, Denmark, Norway and Poland have also been warned.

Candid assessments of foreign leaders

The cables are thought to include candid assessments of foreign leaders and governments and could erode trust in the U.S. as a diplomatic partner.

Crowley said the release will place "lives and interests at risk. We are all bracing for what may be coming and condemn WikiLeaks for the release of classified material. It will place lives and interests at risk. It is irresponsible."

Diplomatic cables are internal documents that would include a range of secret communications between U.S. diplomatic outposts and State Department headquarters in Washington.

The U.S. ambassador to Germany, Philip Murphy, told the German newspaper Bild am Sonntag that it will be difficult to predict the consequences of the leaked documents.

"It will be uncomfortable for my government, for those who are mentioned in our reports, and for me personally as U.S. ambassador to Germany," he said in an interview published Sunday.

WikiLeaks has said the release will be seven times the size of its October leak of 400,000 Iraq war documents, already the biggest leak in U.S. intelligence history.

The U.S. says it has known for some time that WikiLeaks held the diplomatic cables. No one has been charged with passing them to the website, but suspicion focuses on U.S. Army Pfc. Bradley Manning, an intelligence analyst arrested in Iraq in June and charged over an earlier leak.

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