A multi-party delegation will visit Washington DC later this month to make the case for the US to drop extradition proceedings against Julian Assange, ahead of the prime minster's visit to the US in October.

A multi-party delegation of federal MPs and senators will travel to Washington DC this month as part of the campaign to release WikiLeaks founder Julian Assange.

The United States is seeking to extradite Mr Assange from the United Kingdom on 18 charges related to the publication of thousands of military and diplomatic documents.

He has been detained in the Belmarsh Prison in London for more than four years, and is currently appealing the UK’s decision to agree to his extradition.

The parliamentary delegation will include former Nationals leader Barnaby Joyce, Labor MP Tony Zappia, Liberal senator Alex Antic, independent MP Monique Ryan, and Greens senators David Shoebridge and Peter Whish-Wilson.