The United States has sought to assure a British court that it would not hold Julian Assange at a supermax prison, seeking to overturn a block on his extradition in order to put him on trial for espionage.
The WikiLeaks founder risks up to 175 years in jail if he is sent to the US and convicted.
But a British judge blocked his extradition on the grounds that he was at serious risk of suicide and that his mental health would probably deteriorate in the US penal system.
At the start of a two-day appeal hearing in London, the US government asked Britain's High Court to overturn the ruling from January, arguing the judge had been "wrong to come to the conclusion she did".
The initial judgment was partly based on concerns Mr Assange was likely to be held in isolation in the US's only federal so-called supermax prison, the ADX Florence jail in Colorado.
James Lewis, lawyer for the US government, told the court that Washington had since provided written assurances that Mr Assange would not be held at the notorious prison, either pre-trial or afterwards.
Julian Assange "will receive any such clinical and psychological treatment as is recommended" and that he will eventually be eligible to apply for a prisoner transfer to his native Australia, Mr Lewis added.
"There has never been a previous breach of an assurance," he argued.
Mr Assange's team hit back that the assurances do not rule out altogether the chance of him being detained at the facility, at a "comparable" federal prison or at a state-level supermax jail.
The new US pledges provided "no reliable basis" for changing the lower court judge's decision, they said in legal filings.
The two senior judges hearing the appeal at the High Court will deliver their own ruling at a later date, but the legal fight is likely to drag on for months if not years.
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Outside the court in central London, Julian Assange's partner Stella Moris, with whom he has two children, joined two dozen protesters demanding his immediate release from London's high-security Belmarsh jail.
"I'm very concerned for Julian's health," Ms Moris told reporters.
"He is very thin. And I hope that the courts will end this nightmare," she said, having visited Mr Assange in prison on Saturday.
One demonstrator wearing a funereal veil held a banner reading "RIP British Justice". Another, 20-year-old Ruby Allen, said Julian Assange was an innocent defender of "press freedom".
"Extradition is a death sentence basically," she said.
The US government wants Julian Assange, 50, to face 18 charges relating to WikiLeaks' 2010 release by 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 aid he gave former military intelligence officer Chelsea Manning in obtaining the documents from secure computer systems.
The US government has said that the British judge who ruled in January, Vanessa Baraitser, "didn't appreciate the weight" of expert evidence that said Assange was not at risk of suicide.
Judge Baraitser 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.
Mr Lewis noted Julian Assange had "no history of serious and enduring mental illness" and cautioned against a "crystal ball" approach to what could happen if he was extradited.
He said the earlier ruling focused on the intellectual ability of an individual such as Mr Assange to "get around suicide prevention measures".
"That then becomes a trump card that cannot be dealt with by the requesting state," the lawyer added.
Julian Assange 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.
Whatever the High Court decides, further legal wrangling looms.
If the US appeal is successful, the case will be sent back to a lower court for a new decision.
Whoever loses can also ask for permission for a further, final appeal to the UK's Supreme Court.
Mr 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.