WikiLeaks said it had to fight off a massive cyber attack overnight as it began posting hundreds of thousands of classified US diplomatic documents.

Hours before the planned release of the US embassy cables, WikiLeaks said on its Twitter feed that it was "under a mass distributed denial of service attack".

From around 5:00pm GMT (2:00am AEDT) its main website, at http://wikileaks.org, was inaccessible due to the attack.

But after creating a sub-website - http://cablegate.wikileaks.org - the whistleblower group went ahead with the publishing of the documents after US, British, Spanish, French and German newspapers also followed through with their own account of the leak.

"Tomorrow we will provide information on how other media groups can apply for embargo access to #cablegate info," WikiLeaks said later in a statement on its Twitter feed.

The main WikiLeaks website was still apparently under a distributed-denial-of-service (DDoS) attack this morning and was not immediately accessible.

Classic DDoS attacks occur when legions of "zombie" computers, normally machines infected with viruses, are commanded to simultaneously visit a website.

Such a massive onslaught of demand can overwhelm servers, slowing service or knocking it offline completely.

It was unclear where the DDoS attack on WikiLeaks had originated.

Its decision to publish the trove had faced stiff opposition from all parts of the US diplomatic and military apparatus.

WikiLeaks said on its "cablegate" website that the 251,287 documents - including 15,652 cables classified as secret - date from 1966 to February this year and contain "confidential communications between 274 embassies in countries throughout the world and the State Department in Washington DC."

WikiLeaks said it had so far released 219 documents online.

- AFP