The US has written to the founder of whistle-blowing website Wikileaks, Julian Assange, urging him not release a batch of diplomatic files.
The release of classified state department documents is against US law and will put "countless" lives at risk, the letter warns.
Wikileaks says it is set to unveil a new set of documents, bigger than past releases on Afghanistan and Iraq.
Mr Assange has said the US authorities are afraid of being held to account.
The latest leak is expected to include documents covering US dealings and diplomats' confidential views of countries including Australia, Britain, Canada, Israel, Russia and Turkey.
"The material that we are about to release covers essentially every major issue in every country in the world," Mr Assange told reporters by video link.
A journalist with Britain's Guardian newspaper, which has been working with Wikileaks on publishing the files, said they would include an unflattering US assessment of UK PM David Cameron.
No-one has been charged with passing the diplomatic files to the website but suspicion has fallen on US Army private Bradley Manning, an intelligence analyst arrested in Iraq in June and charged over an earlier leak of classified US documents to Mr Assange's organisation.
'Seven times larger'
The letter from the US state department's legal advisor Harold Koh was a response to correspondence from Mr Assange, who had written to the US ambassador to Britain, Louis Susman.
Mr Assange had asked which individuals would be put at risk by the leak, the state department said.
A senior American official told the BBC that Mr Assange was offering to negotiate over limited redactions.
In response, Mr Koh demanded that Wikileaks return official documents to the US government.
"We will not engage in a negotiation regarding the further release or dissemination of illegally obtained US government classified materials," Mr Koh stated in the letter.
Mr Koh's letter adds that the publication of the documents would endanger the lives of "countless" individuals - from journalists to human rights activists and bloggers - and put US military operations at risk.
Correspondents say the letter is a rare move for the US administration, and reflects the government's concern about the implications of the possible leak.
Wikileaks earlier this week said that the next release would be nearly seven times larger than the nearly 400,000 Pentagon documents related to the Iraq war it published in October.
It has not confirmed when the documents will be made public, but there is some speculation that the release will take place on Sunday. Guardian journalist Simon Hoggart said the paper would publish extensive details on Monday.
He told the BBC: "There is going to be some embarrassment certainly for Gordon Brown but even more so for David Cameron who was not very highly regarded by the Obama administration or by the US ambassador here."
The Sunday Times also quoted an official who warned that British citizens in Muslim countries could be targeted in a backlash against perceived "anti-Islamic" views.
The UK Ministry of Defence has urged newspaper editors to "bear in mind" the national security implications of publishing any of the files.
Wikileaks argues that the site's previous releases shed light on the wars in Afghanistan and Iraq. They included allegations of torture by Iraqi forces and reports that suggested 15,000 additional civilian deaths in Iraq.