A Swedish appeals court has upheld the detention order on Julian Assange, dismissing a challenge by the WikiLeaks founder who is wanted by Swedish prosecutors in an investigation of alleged sex crimes.

Confirming a ruling by a lower court, the Svea appeals court said there is no reason to lift the detention order just because it cannot be enforced at the moment.

Mr Assange has avoided being extradited to Sweden by taking shelter in the Ecuadorean Embassy in London.

The court also criticised the prosecutors, who have declined Mr Assange's offer to be questioned in London, for not considering "alternative avenues" to move the investigation forward.

He has not been formally indicted in Sweden, but he is wanted for questioning by police over allegations of sexual misconduct and rape involving two women he met during a visit to the Scandinavian country in 2010. He denies the allegations.

His lawyers argued that the detention order that underlies Sweden's request for his extradition should be lifted, on the grounds that it cannot be enforced while he is at the embassy and because it is restricting Mr Assange's civil rights.

"In the view of the Court of Appeal there is no reason to set aside the detention solely because Julian Assange is in an embassy and the detention order cannot be enforced at present for that reason," the court said in a statement.

"The reasons for detention still outweigh the reasons to the contrary since Julian Assange is suspected of crimes of a relatively serious nature and there is a great risk that he will evade legal proceedings or punishment if the detention order is set aside," the court added.

Mr Assange's lawyer Per E Samuelson said the defence team would appeal the decision to Sweden's Supreme Court.