New documents from WikiLeaks reveal Labor powerbroker Mark Arbib warned the United States about a possible challenge to Kevin Rudd's prime ministership as early as last year.

Extracts published in Fairfax newspapers show that in October last year, Senator Arbib secretly told US diplomats that Mr Rudd wanted to prevent a challenge from Julia Gillard.

The cables say Senator Arbib told diplomats that Mr Rudd, now Australia's Foreign Minister, wanted ''to ensure that there are viable alternatives to Gillard within the Labor Party to forestall a challenge''.

The cables suggest Senator Arbib has been secretly informing the US embassy in Canberra for several years.

The former Labor minister Bob McMullan and the federal MP Michael Danby have also been revealed as close sources.

An embassy profile written in July 2009 says of Senator Arbib:''He understands the importance of supporting a vibrant relationship with the US while not being too deferential."

"We have found him personable, confident and articulate. He has met with us repeatedly throughout his political rise.''

A spokesman for Senator Arbib says he is known as a strong supporter of Australia's relationship with the US and, like many members of the federal parliament, has regular discussions about the state of Australian and US politics with US diplomats.

Earlier, Mr Rudd said the United States, not WikiLeaks founder Julian Assange, was to blame for the release of secret diplomatic cables.

Mr Assange is in custody in Britain facing extradition to Sweden in relation to sexual assault allegations, but authorities in both countries insist his detention has nothing to do with the recent release of the secret cables.

Mr Assange, who denies the allegations, will remain behind bars until an extradition hearing on December 14.

Mr Rudd says the 39-year-old Australian cannot be held personally responsible for the release of more than 250,000 documents.

He says the leaks raise questions about the adequacy of US security.

"Mr Assange is not himself responsible for the unauthorised release of 250,000 documents from the US diplomatic communications network," said Mr Rudd, who had been criticised in one leaked cable as a "control freak".

"The Americans are responsible for that."

The original source of the leaks is not known, though a US army private who worked as an intelligence analyst in Iraq, Bradley Manning, has been charged by military authorities with unauthorised downloading of more than 150,000 State Department cables.

US officials have declined to say whether those cables are those now being released by WikiLeaks.

- ABC/wires