Image caption Wikileaks has previously released documents relating to Iraq and Afghanistan

Whistle-blowing website Wikileaks says it has come under attack from a computer-hacking operation, ahead of a release of secret US documents.

"We are currently under a mass distributed denial of service attack," it said on its Twitter feed earlier.

It added that several newspapers will go ahead and publish the documents released to them by Wikileaks even if the site goes down.

The US state department has said the release will put many lives at risk.

Wikileaks founder Julian Assange has said the US authorities are afraid of being held to account.

Wikileaks has said the release of classified messages sent by US embassies will be bigger than past releases on Afghanistan and Iraq.

The newspapers set to publish details of the US embassy cables include Spain's El Pais, France's Le Monde, Germany's Spiegel, the UK's Guardian and the New York Times.

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 on Sunday.

Wikileaks Whistle-blowing website with a reputation for publishing sensitive material

Run by Julian Assange, a secretive Australian with a background in computer network hacking

Released 90,000 secret US records of US military incidents about the war in Afghanistan and 400,000 similar documents on Iraq

Also posted video showing US helicopter killing 12 people - including two journalists - in Baghdad in 2007

Other controversial postings include screenshots of the e-mail inbox and address book of US vice-presidential candidate Sarah Palin What is Wikileaks? Profile: Julian Assange

A journalist with Britain's Guardian newspaper said the files include an unflattering US assessment of UK PM David Cameron.

Simon Hoggart 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."

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.

'Illegally obtained'

The US government has written to Mr Assange, urging him not release the documents.

The letter from the US state department's legal adviser Harold Koh said the release of classified state department documents was against US law and would put "countless" lives at risk.

Mr Assange is said to have asked which individuals would be put at risk by the leak and offered 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," he said 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.

Wikileaks earlier this week said that its next release of documents would be nearly seven times larger than the nearly 400,000 Pentagon documents relating to the Iraq war it published in October.

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.