More than 60 doctors have written an open letter saying they fear Julian Assange's health is so bad the WikiLeaks founder could die inside a top-security British jail.

Key points: The doctors are calling for Julian Assange to be moved from Belmarsh prison in London to a university teaching hospital

The doctors are calling for Julian Assange to be moved from Belmarsh prison in London to a university teaching hospital The letter says Mr Assange requires urgent expert medical assessment of both his physical and psychological state

The letter says Mr Assange requires urgent expert medical assessment of both his physical and psychological state The doctors based their assessment partially on "harrowing eyewitness accounts" of his October 21 court appearance

In the letter published on Monday (local time), the doctors say Assange suffers from psychological problems including depression, dental issues and a serious shoulder ailment.

The 48-year-old Australian is still fighting a US bid to extradite him from Britain on charges filed under the espionage act that could see him given a sentence of up to 175 years in an American prison.

In the letter to British Home Secretary Priti Patel the doctors call for Assange to be moved from Belmarsh prison in south-east London to a university teaching hospital.

Australian doctor Lissa Johnson said an independent medical assessment was needed to determine if Assange was "medically fit" to face legal proceedings.

The doctors based their assessment on "harrowing eyewitness accounts" of Assange's October 21 court appearance in London and a November 1 report by Nils Melzer, the United Nations special rapporteur on torture.

The independent UN rights expert said Assange's "continued exposure to arbitrariness and abuse may soon end up costing his life".

WikiLeaks founder Julian Assange appeared confused in his last court hearing. ( Reuters: Henry Nicholls )

"We write this open letter, as medical doctors, to express our serious concerns about the physical and mental health of Julian Assange," the doctors said in their 16-page open letter.

'No time to lose'

They said they had "concerns about Mr Assange's fitness" to go through the full extradition hearing, which is set for February.

"Mr Assange requires urgent expert medical assessment of both his physical and psychological state of health," they wrote.

Mr Assange still fights a US bid to extradite him from Britain on charges that could see him sentenced to up to 175 years in a US prison. ( AP: Kirsty Wigglesworth )

"Any medical treatment indicated should be administered in a properly equipped and expertly staffed university teaching hospital [tertiary care].

"Were such urgent assessment and treatment not to take place, we have real concerns, on the evidence currently available, that Mr Assange could die in prison. The medical situation is thereby urgent. There is no time to lose."

The doctors are from the United States, Australia, Britain, Sweden, Italy, Germany, Sri Lanka, Poland.

Assange seemed frail in a court hearing last month — his first appearance in public for six months.

He also appeared confused whenever he was asked to talk at Westminster Magistrates' Court in London.

He seemed to have difficulties recalling his birth date, and at the end of the hearing told District Judge Vanessa Baraitser that he had not understood what had happened in court.

He also complained about the conditions in which he was being kept in Belmarsh.

It was his first public appearance since being dramatically dragged from Ecuador's embassy in April.

Assange used WikiLeaks to publish classified military and diplomatic files in 2010 about US bombing campaigns in Afghanistan and Iraq that proved highly embarrassing to the US government.

Swedish prosecutors said last Tuesday (local time) they had dropped their investigation into a 2010 rape allegation, even though they found the plaintiff's claim "credible".

AFP/AP