SWEDEN'S Supreme Court has upheld a court order to detain WikiLeaks founder Julian Assange for questioning over allegations of rape and sexual molestation.

The 39-year-old Australian, who denies the accusations made by two Swedish women after his visit to the country in August, had appealed two lower court rulings allowing investigators to bring him into custody and issue an international arrest warrant.

He has not been formally charged, although Interpol has placed him on a wanted list.

WikiLeaks has angered the US and other governments by publishing almost half a million secret documents about US diplomatic relations and the wars in Afghanistan and Iraq.

The US officially opened a criminal investigation after WikiLeaks released tens of thousands of secret documents on US military operations in Afghanistan in July.

But now, after being embarrassed by a third massive document dump - this time a quarter of a million US diplomatic cables, many with candid assessments of world leaders - US officials are under intensifying pressure to act.

The search for Assange, whose whereabouts are unknown, was stepped up yesterday as Sweden confirmed it had issued a European arrest warrant for him.

Since leaving Sweden, the computer hacker has appeared in Britain and Switzerland but disappeared from public view after a November 5 press conference in Geneva. He has spoken publicly only through online interviews.

Assange's attorney in Britain, Mark Stephens said British police and the security services of several nations knew of his client's whereabouts.

"Scotland Yard know where he is, the security services from a number of countries know where is," Mr Stephens said.

"The (British) police are being slightly foxy in their answers, but they know exactly how to get in touch with him, as do the Swedish prosecutors."

Asked about the reports that Assange was in southeast England, he said: "I have not said that. I am not saying where he is."

Swedish police said they would issue a new international arrest warrant for Assange on suspicion of "rape, sexual molestation and unlawful coercion" to replace one that could not be applied in Britain because of a procedural error.

Mr Stephens said he would challenge any eventual British arrest warrant in court.

"The process in this case has been so utterly irregular that the chances of a valid arrest warrant being submitted to me are very small," he said.

Mr Stephens said he wasn't prepared to detail the nature of any possible legal challenge as he had yet to be served with a warrant.

Assange's Swedish lawyer, Bjorn Hurtig, was not available for comment.