Julian Assange has told a London court he will fight extradition to the United States over charges related to leaks of classified government material.

Assange again appealed for Australian diplomatic protections as he began his legal battle against US extradition.

The 47-year-old Wikileaks founder appeared via video link a day after being sentenced to almost a year in prison for skipping bail.

A defiant Assange told Westminster Magistrates’ Court he would not surrender for doing award-winning work that had “protected” people.

“I do not wish to surrender myself for doing journalism that has won many, many awards and protected many, many people,” he said from Belmarsh prison, wearing a black blazer, pale t-shirt, jeans and sneakers.

Assange faces up to five years in a US jail over a charge of conspiring to commit computer intrusion with former US Army intelligence analyst Chelsea Manning between January and May in 2010.

Outside court, his barrister Jennifer Robinson made a fresh appeal for the Australian government to intervene in the case.

“What we would like to see is action at the higher political levels and diplomatic protection being exercised over Julian Assange,” Ms Robinson said.

“We have been asking the Australian government since 2010 to seek assurances to protect him from US extradition, the very matter that he’s facing right now and we would like the Australian government to be raising his case.”

US government legal representatives Ben Brandon earlier told the court that Assange and Manning had made a clandestine deal to try to hack the passwords of Pentagon computers to access classified information.

“The charge relates to one of the largest compromises of classified information in the history of the United States,” he said.

“Evidence obtained in the course of the investigation shows Ms Manning and Mr Assange unlawfully conspired to effectuate this disclosure,” he added.

Ms Robinson said afterwards that there was no suggestion her client had ever hacked US military computers.

She said the case boiled down to Assange being alleged to have had communications with his source Manning, encouraged her to provide more information and conversed with her about protecting her identity.

“Journalists do this all the time, that’s why this indictment and the extradition request is such a concern for free speech groups here (the UK) and in the US, and it’s why Julian is committed to defending himself and resisting extradition in this case,” Ms Robinson said.

Assange was remanded in custody until another procedural hearing in the same court on May 30.

The extradition case is set to begin on June 12, with Mr Brandon wanting an hour for that day.

-with AAP