THE head of the United States Senate's powerful intelligence oversight committee has renewed calls for Julian Assange to be prosecuted for espionage.

The US Justice Department has also confirmed WikiLeaks remains the target of a criminal investigation, calling into question Australian government claims the US has no interest in extraditing Mr Assange.

Julian Assange remains in Ecuador's embassy in London while the Ecuadorian government assesses his application for political asylum. Credit:Reuters

"Mr Assange should be prosecuted under the Espionage Act [of 1917]," the chairman of the Senate Select Committee on Intelligence, Californian Democrat Senator Dianne Feinstein, said in a written statement provided to The Age.

"I believe Mr Assange has knowingly obtained and disseminated classified information which could cause injury to the United States. He has caused serious harm to US national security, and he should be prosecuted accordingly."