THE head of the US 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 an ongoing criminal investigation, calling into question Australian government claims that the US has no interest in extraditing Mr Assange.

''I believe Mr Assange has knowingly obtained and disseminated classified information which could cause injury to the United States,'' the chairwoman of the Senate Select Committee on Intelligence, Dianne Feinstein, said in a written statement provided to the Herald. ''He has caused serious harm to US national security, and he should be prosecuted accordingly.''

Senator Feinstein's call for the Obama administration to move ahead with plans to prosecute Mr Assange came as a US Justice Department spokesman, Dean Boyd, publicly confirmed that ''there continues to be an investigation into the WikiLeaks matter''.