The treatment of WikiLeaks founder Julian Assange, who is facing the threat of extradition from Britain to the US on espionage charges, is putting his life "at risk", according to an independent UN rights expert.

Key points: Julian Assange "continues to be detained under oppressive conditions of isolation and surveillance", according to Nils Melzer

Julian Assange "continues to be detained under oppressive conditions of isolation and surveillance", according to Nils Melzer Mr Melzer visited Mr Assange in May but he said he has new "medically relevant information" received from "reliable sources"

Mr Melzer visited Mr Assange in May but he said he has new "medically relevant information" received from "reliable sources" He also said Mr Assange had limited access to legal counsel

"Unless the UK urgently changes course and alleviates his inhumane situation, Mr Assange's continued exposure to arbitrariness and abuse may soon end up costing his life," the UN special rapporteur on torture and other cruel, inhuman or degrading treatment, Nils Melzer, said in a statement.

Mr Melzer, who visited the 48-year-old Australian 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, has previously warned he was being subjected to drawn-out "psychological torture".

The independent expert, who does not speak on behalf of the United Nations, has not met Mr Assange since.

But he told the AFP news agency in an email that his growing concern was based on "new medically relevant information received from several reliable sources."

This information, he said, indicated that, "Mr Assange's health has entered a downward spiral of progressively severe anxiety, stress and helplessness typical for persons exposed to prolonged isolation and constant arbitrariness."

'Life-threatening situation'

Julian Assange is wanted in the US over charges he violated the US Espionage Act. ( PA: Victoria Jones via AP )

"While the precise evolution is difficult to predict with certainty, this pattern of symptoms can quickly develop into a life-threatening situation involving cardiovascular breakdown or nervous collapse," he warned.

Mr Melzer pointed out that in May he had demanded London immediately take measures to protect Mr Assange's health and dignity.

"However, what we have seen from the UK Government is outright contempt for Mr Assange's rights and integrity," he said.

"Despite the medical urgency of my appeal, and the seriousness of the alleged violations, the UK has not undertaken any measures of investigation, prevention and redress required under international law."

Mr Assange "continues to be detained under oppressive conditions of isolation and surveillance, not justified by his detention status," he added.

His statement pointed out that Mr Assange had completed his prison sentence for violating his British bail terms in 2012 and was now "being held exclusively in relation to the pending extradition request from the United States."

Mr Assange is wanted in the US over charges he violated the US Espionage Act by publishing a huge cache of military and diplomatic files in 2010.

"While the US Government prosecutes Mr Assange for publishing information about serious human rights violations, including torture and murder, the officials responsible for these crimes continue to enjoy impunity," Mr Melzer said.

"Despite the complexity of the proceedings against him led by the world's most powerful government, Mr Assange's access to legal counsel and documents has been severely obstructed."

This, he said, had effectively undermined "his most fundamental right to prepare his defence".

Mr Melzer urged London to bar Mr Assange's extradition to the US, and demanded that "he be promptly released and allowed to recover his health and rebuild his personal and professional life."

AFP