WikiLeaks founder Julian Assange says he has heard rumours he has been indicted in the United States for espionage.

Mr Assange, who was freed on bail in England overnight, said he feared efforts were under way to move him into a jurisdiction from where he could more easily be extradited to the US.

"We have also heard today from one of my US lawyers, yet to be confirmed ... that there may be a US indictment for espionage for me coming from a secret US grand jury investigation," the 39-year-old Australian told Sky News.

Mr Assange was arrested and remanded in custody on December 7 in London at the request of Swedish authorities, which want the WikiLeaks founder extradited in order to question him over allegations of sexual offences. He denies any wrongdoing.

Speaking this morning after his arrival at Ellingham Hall, a friend's country mansion in Suffolk, eastern England, Mr Assange expressed fears the extradition proceedings to Sweden were a cover to get him to the United States.

He said the extradition proceeding to Sweden was "occurring in a very strange and unusual way".

This, he feared, might be "actually an attempt to get me into a jurisdiction which will then make it easier to extradite me to the US".

His lawyer, Mark Stephens, claimed earlier this week a secret US grand jury had been set up in Virginia to work on charges that could be filed against the WikiLeaks founder.

Speaking to reporters earlier, he said: "I do not have too many fears about being extradited to Sweden. There are much bigger concerns about being extradited to the United States."

His lawyers insist moves to extradite him to Sweden to face questioning over allegations he sexually assaulted two women are politically motivated and linked to the US cables.

Swedish prosecutors insist the extradition request has nothing to do with WikiLeaks, which is currently releasing 250,000 secret US diplomatic cables.

- AFP