The US has written to the founder of whistleblower site Wikileaks, Julian Assange, requesting him not to release a cache 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 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 due to 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 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.

Analysts say the US and its allies have the potential to be embarrassed by the publication of candid assessments of foreign governments by its officials.

The British media said its government was bracing for the release, with the Sunday Times quoting 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.

The US government has already briefed a number of foreign governments, including the UK, about the issue.

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.

Whistle-blowing website Wikileaks has now released 250,000 secret messages sent by US embassies which give an insight into current American global concerns.

From Dr. Lawrence Britt's Fourteen Defining Characteristics of Fascism:

6. Controlled Mass Media - Sometimes to media is directly controlled by the government, but in other cases, the media is indirectly controlled by government regulation, or sympathetic media spokespeople and executives. Censorship, especially in war time, is very common.

7. Obsession with National Security - Fear is used as a motivational tool by the government over the masses.