The US government will on Wednesday appeal against a British judge's decision to block the extradition of WikiLeaks founder Julian Assange to face trial for publishing military secrets.

At a two-day hearing, Washington will ask the High Court to overturn District Judge Vanessa Baraitser's January ruling that Assange is a serious suicide threat if extradited across the Atlantic.

The United States has said it was "extremely disappointed" at her decision, arguing the judge "didn't appreciate the weight" of expert evidence that said Assange was not at risk of suicide.

Its lawyers have argued Baraitser was "misled" in evidence from Assange's psychiatric expert Michael Kopelman, who they claim concealed things such as that his client had fathered children while holed up in the Ecuadorian embassy in London.

During a preliminary hearing in August, the High Court granted the US government's request to appeal against the ruling on five grounds.

Whatever the court's two senior judges decide, months if not years of further legal wrangling loom.

If the US appeal is successful, the case will be sent back to a lower court for a new decision. And whoever loses can also ask for permission for a further, final appeal to the UK's Supreme Court.

- Indicted -
Assange, 50, was arrested in Britain in 2019 for jumping bail after spending seven years inside the Ecuadorian embassy to avoid extradition to Sweden where he faced allegations of sexual assault. These were later dropped.

Despite his extradition being blocked, he has been denied bail pending the outcome of the US appeal, amid fears he would abscond. 

He is being held at London's high-security Belmarsh Prison, which his fiancee Stella Moris -- a former member of his legal team who is the mother of his two young boys -- this week branded "a terrible environment".

Assange is wanted in Washington to face 18 charges relating to the 2010 release by WikiLeaks of 500,000 secret files detailing aspects of military campaigns in Afghanistan and Iraq.

He has been indicted for violating the US espionage act and for hacking, based on the alleged assistance he provided former military intelligence officer Chelsea Manning in obtaining the documents from secure military computer systems.

If convicted in the US, he faces a maximum sentence of 175 years in jail.

Baraitser had said it was not clear that Washington would be able to ensure his safety in US prisons known for "harsh conditions" while he awaited trial. 

She rejected US experts' testimony that Assange would be protected from self-harm, noting that others such as disgraced US financier Jeffrey Epstein had managed to kill themselves in custody.

- 'Not survive' -
Australian national Assange has a vocal campaign of supporters, led by Moris and his legal team.

Moris said on Monday that she had visited Assange in prison last weekend and was shocked by how thin he looked.

"He was looking very unwell," she said. "The point was that Julian would not survive extradition, that was the conclusion of the judge."

Legal expert Carl Tobias, from the University of Richmond in Virginia, said there was "some chance" of the US winning its appeal.

"The US may be able to convince the High Court that Baraitser assigned too much weight" to the Kopelman report, he told AFP.

"However, even if the High Court agrees with the US contention that she accorded too substantial weight to the expert report, that may not be sufficient to warrant overruling her entire decision," he added.

Advocacy group Reporters Without Borders has urged President Joe Biden to drop the case, arguing the WikiLeaks founder has been "targeted for his contributions to public-interest reporting".

Assange sought, but failed, to obtain a pardon from Biden's predecessor Donald Trump, whose 2016 election campaign benefited from WikiLeaks' release of materials that damaged his Democratic opponent, Hillary Clinton.