Julian Assange faces the prospect of being denied press protections under US law if he goes to trial there, WikiLeaks says, citing evidence submitted for his London extradition case.

The 48-year-old WikiLeaks founder is set to face trial in the UK next month to determine whether he should be extradited to the US, where he has been charged with 17 counts of spying and one count of conspiracy to commit computer intrusion.

The charges related to allegations Assange tried to help former US army intelligence analyst Chelsea Manning protect her digital identity as she accessed classified Pentagon files on the Iraq and Afghanistan wars.

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WikiLeaks helped publish thousands of those files, including some that revealed US war crimes in both countries. His case is widely viewed as a litmus test for the protection of journalists' sources.

READ MORE Who is Chelsea Manning and why might Australia ban her?

WikiLeaks editor Kristinn Hrafnsson says a new affidavit provided by US government lawyers this week for Assange's upcoming extradition trial states that foreign nationals, like Assange, are not entitled to press protections under the US Constitution's First Amendment.

Mr Hrafnsson revealed the development outside Assange's case management hearing at London's Westminster Magistrates Court on Thursday.

"On the one hand they have decided that they can go after journalists wherever they are residing in the world, they have universal jurisdiction, and demand extradition like they are doing by trying to get an Australian national from the UK for publishing that took place outside US borders," he told AAP.

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"But then at the same time, they are not granting any foreign journalist the protection of the First Amendment.

"That's extremely serious. That's of grave concern to all journalists.

"We are seeing this incremental approach, a darkness flowing over journalism from that country, and it's about time that journalists really united in resisting this."

Assange appeared by video link from prison at Thursday's hearing and did not speak except to say his name and birthdate for the court.

Judge Vanessa Baraitser reluctantly agreed to split his trial into two segments with the first week to begin on 24 February and the final three weeks to be held from 18 May.

Her ruling came after prosecutors flagged timetabling issues and the defence pleaded for more time to deal with an ever-expanding pile of evidence coming from the US.

Mr Hrafnsson says the delay may give Assange and his legal team more time to review mounting evidence, as they have only been permitted four hours together since his arrest on 11 April.

But he admitted it would also further extend Assange's time behind bars.

"A maximum-security prison for a non-violent person like Julian, who is a free man basically, who is on remand, is outrageous," Mr Hrafnsson told AAP.

"It's totally unacceptable."