Supporters of Julian Assange hold placards in protest outside Westminster Magistrates Court in London. Tolga Akmen/AFP

Julian Assange has been subjected to drawn-out "psychological torture", a UN rights expert said Friday, accusing the United States, Britain, Ecuador and Sweden of "collective persecution" of the WikiLeaks founder.

The United Nations special rapporteur on torture and other cruel, inhuman or degrading treatment, Nils Melzer, also warned that if London agrees to an extradition request from Washington, Assange risked the death penalty.

Melzer visited the Australian whistleblower in a London prison on May 9, nearly a month after his arrest at Ecuador's embassy where he had been holed up for seven years.

Julian Assange. File Photo

"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," Melzer said in a statement.

"Most importantly, 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," he said.

Melzer, who was accompanied on the visit by two medical experts specialised in examining potential torture victims, said there was "overwhelming" evidence that Assange had been "deliberately exposed, for a period of several years, to progressively severe forms of cruel, inhuman or degrading treatment or punishment.

"The cumulative effects," he said, "can only be described as psychological torture."

Assange, 47, sought refuge at Ecuador's embassy in 2012 to avoid extradition to Sweden, where he faces accusations of sexual assault.

He is currently serving a separate 50-week sentence at London's Belmarsh prison for skipping bail.

Assange now also has an extradition request against him from the US, which has charged him with violating the US Espionage Act by publishing a huge cache of military and diplomatic files in 2010, rejecting his claim that he is a journalist.

Agence France-Presse