The US government made a fresh extradition request to have Assange sent from the UK based on charges in a new superseding indictment.

Assange, 49, faces 17 charges under the US espionage act and conspiracy to commit computer intrusion.

He is accused of working with former US army intelligence analyst Chelsea Manning to leak hundreds of thousands of classified documents.

A hearing at Westminster Magistrates' Court on Friday was told that the United States sought to expand alleged conduct within the charges through a third version of the criminal indictment against him.

Prosecutor Claire Dobbin explained this included extending the group of people Assange is alleged to have conspired with beyond Manning.

The WikiLeaks founder has yet to be arrested over the latest indictment, full details of which were not read out in court.

Assange's extradition hearing, originally set for May, was postponed as a result of the coronavirus lockdown measures and was rescheduled for September 7 at the Old Bailey.

Florence Iveson, representing Assange in court on Friday, said US prosecutor Gordon Kromberg had submitting a 33-page additional affidavit explaining the second superseding indictment on August 12.

She said the "11th hour" submission of the document was "astonishing", claiming the US was "seeking to add a considerable amount of conduct and seeking to extend the case significantly".

Iveson said lawyers had not been able to take full instructions from Assange over the third version of the indictment against him.

Assange, who spoke only to give his name, date of birth and to confirm he had heard the judge, attended the short hearing via video-link from Belmarsh prison where he is being held.

His father, John Shipton, watched the proceedings from the public gallery, alongside other supporters of the WikiLeaks founder.

Iveson argued the new material should have been provided "at a much earlier stage", adding it could affect the evidence of witnesses due to be called in September.

She called on district judge Vanessa Baraitser to "exclude" the new alleged conduct within the charges, claiming it was the only way "to achieve justice".

Dobbin said Kromberg had explained how the US had continued to conduct investigations into alleged criminal conduct by Assange that had not been included in the earlier indictments.

"It's lawful and common for US prosecutors to continue investigating a defendant's criminal conduct after he's been arrested and charged," she told the court.

"He's explaining how it is that in the American system there can be superseding indictments."

Dobbin said all of the alleged conduct in the first superseding indictment was also in the second, but it added "further parties" to the conspiracy allegations, beyond Manning.

She said the new indictment "does not fundamentally alter the basis upon which extradition is sought".

Judge Baraitser highlighted that Assange "has not yet even been arrested" in relation to the new extradition request and further delay to his case "should be avoided".

She told the court she did not have the case management powers to handle the defence's exclusion request and ruled it was up to Assange's legal team to decide whether to make an application to vacate the case to give them more time, despite the consequences being "extremely unattractive".

Iveson said she and her colleagues would use "best endeavours" to make their decision by August 19 but warned they may need until August 21.

Adjourning proceedings, Judge Baraitser told Assange that depending on his lawyers' actions this was the last administrative hearing for his case.

She said he is due to be physically present when his full extradition hearing takes place next month.

Australian Associated Press