The plan for Swedish prosecutors to question WikiLeaks founder Julian Assange at Ecuador's embassy in London has stalled.

A Swedish official said the questioning was postponed after Ecuador demanded Sweden give him asylum as a condition of the meeting.

"You can't give anyone asylum at another country's embassy, that's against international law," Swedish justice department official Cecilia Riddselius said.

"If he wants asylum, he has to come to Sweden."

Assange took refuge in the Ecuadorian embassy in June 2012 to avoid extradition to Sweden, where he is wanted for questioning over allegations of sexual assault and rape against two women in 2010.

He denies the accusations.

Assange said he feared Sweden would extradite him to the United States where he could be put on trial over WikiLeaks' publication of classified military and diplomatic documents five years ago — one of the largest information leaks in US history.

Prosecutors first insisted Assange should travel to Sweden for questioning but in a U-turn in March agreed to conduct the interview in London.

In early June, prosecutors submitted a request for legal assistance to UK authorities and a request to Ecuador for permission to interview Assange.

Ms Riddselius said the justice department had done all it could to allow the questioning to take place and the matter was now "completely in Ecuador's hands".

A Swedish prosecution authority spokesman said a prosecutor went to London ready to question the Australian but the embassy did not allow her to enter and she had to return home.

The embassy was not immediately available for comment.

Reuters