Lawyers for Julian Assange on Thursday dismissed US assurances about the treatment awaiting the WikiLeaks founder if he is extradited from Britain, as two days of hearings wrapped up in London.

Britain's High Court said it would issue a ruling at a later date, after Washington appealed against a lower court's decision to block Assange's extradition to face a series of US charges related to the mass leak of classified documents.

"You've given us much to think about and we will take our time to make our decision," said Ian Burnett, one of the two judges hearing the US appeal in the central London court.

Assange's lawyers argued that he remains a suicide risk if extradited to the US, despite new assurances that he would not be held in punishing isolation at a "supermax" federal prison.

Assange faces 18 charges in the US including espionage and hacking Assange faces 18 charges in the US including espionage and hacking Photo: AFP / Tolga Akmen

Mark Summers, representing Assange, argued there were "genuine questions" over the "trustworthiness" of the US pledges.

He said US intelligence agencies had an "obsession" with Assange.

Recent reports that the CIA had hatched a prior plot to kidnap Assange from the Ecuadorian embassy in London and poison him were "potentially the tip of the iceberg", Summers said.

Timeline on Julian Assange, the Australian founder of WikiLeaks. Timeline on Julian Assange, the Australian founder of WikiLeaks. Photo: AFP / Sophie RAMIS

The US government wants Assange to face espionage charges that could put him in jail for up to 175 years, although its legal team claims his possible sentence is difficult to estimate and could be far shorter.

It is appealing against UK district court judge Vanessa Baraitser's decision in January that it would be "oppressive" to extradite Assange because of his serious risk of suicide and mental health deterioration.

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

Whatever the High Court decides, the legal fight is likely to drag on for months if not years.

Assange has fathered two children with Stella Moris in recent years Assange has fathered two children with Stella Moris in recent years Photo: AFP / JUSTIN TALLIS

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

Assange chose not to appear Thursday after following some of Wednesday's proceedings via video-link from the high-security Belmarsh jail in southeast London where he is being held.

His partner Stella Moris, with whom he has two children, was present inside the courtroom as dozens of supporters rallied outside.

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

The US government has indicted him on 18 charges relating to WikiLeaks' 2010 release of 500,000 secret files detailing aspects of military campaigns in Afghanistan and Iraq.

He is accused of violating the US espionage act and hacking, based on the alleged aid he gave former military intelligence officer Chelsea Manning in obtaining the documents from secure computer systems.

Jeremy Corbyn, the left-wing former leader of Britain's Labour party, said outside the court that Assange had "told us the truth, the truth about Afghanistan, the truth about Iraq, the truth about surveillance".

James Lewis, lawyer for the US government, said in its appeal that Washington had now provided written pledges Assange would not be detained at the ADX Florence jail in Colorado, which houses criminals including Al-Qaeda extremists in near-total isolation.

He would also receive any psychological treatment recommended, and eventually be eligible to apply for a prisoner transfer to his native Australia.

"Diplomatic assurances are a solemn matter," Lewis said. "These are not dished out like smarties."

He also sought to undermine Baraitser's ruling, arguing Assange "had every reason to exaggerate" his mental health issues and that its own experts had found he was only "moderately depressed".

The lawyer also insisted Michael Kopelman, a key psychiatric expert provided by Assange's team, had offered a "misleading" initial report which deliberately omitted that Assange had secretly fathered two children with Moris in recent years.