WikiLeaks founder Julian Assange has suffered "psychological torture" from a defamation campaign against him by the media, judges and senior political figures, a United Nations human rights investigator says.

Key points: Assange's condition was assessed by UN rights investigator Nils Melzer and two medical professionals

Assange's condition was assessed by UN rights investigator Nils Melzer and two medical professionals Mr Melzer said Assange showed all symptoms typical of prolonged exposure to psychological torture

Mr Melzer said Assange showed all symptoms typical of prolonged exposure to psychological torture The United States is seeking the extradition of Assange

Nils Melzer, the UN special rapporteur on torture who visited Assange in a high-security London prison on May 9, voiced concern at fresh United States criminal charges laid against Assange and reiterated a call for him not to be extradited.

Assange was too ill on Thursday (local time) to appear via video link from a British prison in a hearing on an extradition request from the US, his lawyer Gareth Peirce said.

He is in a health ward.

"Mr Assange has been deliberately exposed, for a period of several years, to progressively severe forms of cruel, inhuman or degrading treatment or punishment, the cumulative effects of which can only be described as psychological torture," Mr Melzer said in a statement.

The Swiss law professor, an independent expert, was accompanied on the prison visit by two medical experts specialised in examining potential victims of torture and other ill-treatment, the statement said.

They spoke with Assange in private and conducted a "thorough medical assessment".

"It was obvious that Mr Assange's health has been seriously affected by the extremely hostile and arbitrary environment he has been exposed to for many years," Mr Melzer said.

"In addition to physical ailments, Mr Assange showed all symptoms typical for prolonged exposure to psychological torture, including extreme stress, chronic anxiety and intense psychological trauma."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 12 seconds 2 m 12 s Who is Julian Assange and what is WikiLeaks?

The US is seeking the extradition of Assange, who was dragged from the Ecuadorean embassy in London on April 11 after his seven-year asylum was revoked, for one of the biggest-ever leaks of classified information.

The Australian, now 47, had skipped bail and taken refuge there in 2012 to avoid extradition to Sweden over a sexual assault investigation that was later dropped.

Sweden reopened the investigation in early May.

Assange denies the rape allegation.

The US has charged Assange with espionage, saying he unlawfully published the names of classified sources and conspired with and assisted ex-Army intelligence analyst Chelsea Manning in obtaining access to classified information.

He faces 18 US criminal counts and decades in prison if convicted.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 50 seconds 50 s Wikileaks founder Julian Assange is arrested at the Ecuadorian embassy in London.

"My most urgent concern is that, in the United States, Mr Assange would be exposed to a real risk of serious violations of his human rights, including his freedom of expression, his right to a fair trial and the prohibition of torture and other cruel, inhuman or degrading treatment or punishment," Mr Melzer said.

Assange made international headlines in 2010 when WikiLeaks published a classified US military video showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff.

"Since [2010], there has been a relentless and unrestrained campaign of public mobbing, intimidation and defamation against Mr Assange, not only in the United States, but also in the United Kingdom, Sweden and, more recently, Ecuador," Mr Melzer said, pointing to the press and social media, and unnamed senior political figures and judicial magistrates involved in the case.

Reuters