London (AFP) - WikiLeaks founder Julian Assange faces a weeks-long wait to learn if he can make a last-ditch appeal against extradition to the United States, after a UK court on Tuesday delayed a decision.

The High Court in London gave the US government three weeks to provide further "assurances" on his treatment if he is sent there to face charges over Wikileaks' 2010 release of secret military and diplomatic files.

Washington has spent several years trying to extradite the 52-year-old Australian citizen to stand trial for the publication, which related to the US-led wars in Iraq and Afghanistan.

Attempting to halt the process, Assange had suffered a string of court losses in the long-running legal saga, which his supporters see as a battle for media freedom.

A two-day hearing was held last month and two judges in London ruled Tuesday he had "a real prospect of success" on three of his nine grounds of appeal.

However, Victoria Sharp and Jeremy Johnson said Washington had until April 16 to allay concerns that his trial would be prejudiced because he is not a US citizen and that he could face the death penalty if convicted.

"Before making a final decision on the application for leave to appeal, we will give the respondent an opportunity to give assurances," the pair wrote in their 66-page ruling.

"If assurances are not given then we will grant leave to appeal without a further hearing." 

They also noted that if the United States does respond, further submissions could be made and a hearing will be held on May 20 when Assange could learn of his fate.

'Astounding'

The ruling, which was released online, means Assange will remain at the high-security prison in southeast London where has been held since 2019.

He did not attend February's proceedings either in person or remotely due to illness.

If he is eventually granted another appeal, the case will be heard in a London court.

But if the judges are swayed by the US reassurances, Assange will have exhausted all UK appeals.

His team has previously indicated, however, that they will ask the European courts to intervene and that they would be given 14 days to do so.

Assange's wife Stella, who has led the campaign to block his extradition, called the latest decision "astounding".

"What the courts have done is to invite political intervention from the United States, to send a letter saying 'it is all OK'," she told reporters outside the court.

Campaign groups including Amnesty International and Reporters Without Borders also called for Assange's release.

The UN special rapporteur on torture, Alice Jill Edwards, said it was "regrettable" the High Court had not fully ruled on the "disproportionate penalty" Assange faced in the United States.

She said a decades-long jail sentence or ongoing solitary confinement "could amount to inhuman treatment".

Legal saga

The US indicted Assange multiple times between 2018 and 2020 but US President Joe Biden has faced persistent domestic and international pressure to drop the case filed under his predecessor Donald Trump.

Major media organisations, press freedom advocates and the Australian parliament have all denounced the prosecution under the 1917 Espionage Act, which has never been used over the publishing of classified information.

Washington alleges that Assange and others at WikiLeaks recruited and agreed with hackers to conduct "one of the largest compromises of classified information" in US history.

Lawyers for Assange have argued the charges were "political" and that he was being prosecuted "for engaging in ordinary journalistic practice of obtaining and publishing classified information".

They have also raised a host of other concerns.

Before going to prison, Assange spent seven years holed up in Ecuador's London embassy to avoid extradition to Sweden, where he faced accusations of sexual assault which were later dropped. 

The High Court had blocked his extradition, but then reversed the decision on appeal in 2021 after the United States vowed not to imprison him in its most extreme prison, "ADX Florence".

It also pledged not to subject him to the harsh regime known as "Special Administrative Measures", and to allow him to eventually serve out his sentence in Australia.

In March 2022, the UK Supreme Court refused permission to appeal, arguing Assange failed to "raise an arguable point of law".

Months later, ex-interior minister Priti Patel formally signed off on his extradition.

Assange is seeking permission to review that decision and the 2021 appeal ruling.