THE United States has denied it is secretly preparing legal action against Julian Assange as supporters of the WikiLeaks founder explore fresh legal moves to stop his extradition to Sweden to face questioning over rape allegations.

The US ambassador to Australia, Jeffrey Bleich, yesterday rejected as ''an invention'' claims that Washington was preparing a warrant for the arrest of Mr Assange over WikiLeaks' role in publishing thousands of secret US diplomatic cables last year.

''There is no such thing as a secret warrant. Period. They don't exist,'' Mr Bleich said.

On Wednesday, Mr Assange lost a British Supreme Court appeal that sought to prevent him being sent to Sweden.

But his legal team was granted 14 days to submit an application to reopen the appeal after it was argued a majority of the judges had based their decision on an interpretation of the Vienna Convention on the Law of Treaties, a point of law not argued during the appeal hearing.