Attorney Mark Stephens told the weekly Die Zeit in an interview to be published on Thursday that he believed Swedish officials were cooperating with US authorities to extradite Assange as soon as the Americans have built a criminal case against him.
Stephens called the Swedish charges against his client a "holding case" to buy time until the US can prosecute him themselves over Leaks">WikiLeaks' mass release of classified US documents.
A spokesman for the Swedish justice ministry sharply denied the allegations.
"That's a lie. It is not true. There are no negotiations [with the US] in that field," Justice Minister Beatrice Ask's spokesman Martin Valfridsson told AFP.
Valdfridsson reiterated that a Swedish prosecutor wanted to question Assange over allegations of sex offences and that if she determined there were grounds for a trial, those offences "would be the only thing that would be tried."
Assange's lawyer added his client did not believe he would receive a fair trial in Sweden, which was why he was fighting his extradition from Britain.
The Australian has been living at a supporter's country estate in England since being released on bail on December 16th after his arrest by British police on a Swedish warrant.
Stephens said that he believed the "last station" of an extradition to Sweden would be "a high-security prison in the US."
Assange's lawyers released documents Tuesday saying that if the Australian is extradited to Sweden, there is a "real risk" he will face extradition or illegal rendition to the US, where he could be detained at Guantanamo Bay or elsewhere and subject to the death penalty.
A British judge ruled Tuesday that Sweden's bid to have him extradited would be heard in full on February 7th to 8th.
Swedish authorities want to question Assange about charges brought by two women that he sexually assaulted them, but the 39-year-old says the extradition attempt is politically motivated and linked to WikiLeaks' activities.
The whistleblowing website has released classified documents about the wars in Iraq and Afghanistan and from US diplomats stationed around the world.
A US court has reportedly subpoenaed the Twitter accounts of four WikiLeaks supporters as part of a widening criminal investigation into the leaks.