London (CNN) Wikileaks founder Julian Assange shows all the symptoms of psychological torture and risks having his human rights violated if he is extradited to the US, a United Nations expert has said.

Nils Melzer, the UN's Special Rapporteur on torture, said Assange is displaying "extreme stress, chronic anxiety and intense psychological trauma" after being subjected to several years' worth of "progressively severe forms of cruel, inhuman or degrading treatment."

The Australian is battling multiple charges from the United States, including 17 counts under the Espionage Act , and did not appear at a court hearing on Thursday amid concerns about his health.

Melzer met with Assange on May 9 in Belmarsh Prison in London, and conducted a medical on the publisher. Weeks earlier, Assange was arrested after spending seven years living inside the Ecuadorian Embassy in London.

"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.

He added Assange was subjected to "oppressive isolation, harassment and surveillance inside the embassy," as well as "deliberate collective ridicule, insults and humiliation." Assange appeared frail and bearded as he was dragged by police from the embassy, having had virtually no access to sunlight during his lengthy stint there.

Melzer added that Assange's visits from his lawyers are being kept short and infrequent, and he is unable to access legal documents and case files, making it "impossible" for him to prepare his legal defense properly.

On Thursday, Wikileaks expressed "grave concerns" about Assange's health and said he had been moved to the medical wing of his prison.

Alongside his attack on Assange's treatment, Melzer has sent a letter to the British government appealing for them to block the publisher's extradition to the US.

"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," he said.

Assange could face up to 175 years in prison if found guilty of the Espionage Act offenses, for his alleged role in unlawfully encouraging, receiving and publishing national defense information in concert with former US Army intelligence analyst Chelsea Manning. Melzer suggested he could also face the death penalty if further charges were added in the future.

Barry Pollack, an attorney for Assange, said last week that the charges under the Espionage Act sought to punish him "for encouraging sources to provide him truthful information and for publishing that information."

"These unprecedented charges demonstrate the gravity of the threat the criminal prosecution of Julian Assange poses to all journalists in their endeavor to inform the public about actions that have been taken by the US government," Pollack said in a statement.

Assange's legal troubles also include a computer hacking charge, as well as an allegation of rape in Sweden, which he has denied. Authorities there re-opened an investigation into the case earlier this month.

"In 20 years of work with victims of war, violence and political persecution I have never seen a group of democratic states ganging up to deliberately isolate, demonize and abuse a single individual for such a long time and with so little regard for human dignity and the rule of law," Melzer said.

The Australian is currently serving a jail sentence of almost a year in London for skipping bail in 2012 when he sought political asylum in the Ecuadorian embassy.

In a statement sent to CNN, a UK government spokeswoman said: "The UK has a close working relationship with UN bodies and is committed to upholding the rule of law. We support the important work of the Special Rapporteur's mandate and will respond to his letter in due course, but we disagree with a number of his observations.

"Judges are impartial and independent from Government, with any judgment based solely on the facts of the case and the applicable law. The law provides all those convicted with a right of appeal."