The father of WikiLeaks founder Julian Assange said on Friday he worried his son would die in prison after nine years of "persecution" for daring to reveal US "war crimes".

John Shipton told reporters in Geneva that he had visited his son in a British prison two days ago and needed to "face the bitter truth" that he "may die in jail".

"This is not the bitter disappointment of a father, this is simply fact," he said.

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.

Since then, he has been entangled in a web of judicial proceeding, and is currently fighting a US bid to extradite him from Britain on charges filed under the Espionage Act that could land him a sentence of up to 175 years in a US prison.

The 48-year-old whistleblower is currently being held at a top-security British prison since April after police sensationally dragged him out of the Ecuadoran embassy in London.

He had been holed up in the embassy since 2012 to avoid a extradition order to Sweden, where he was wanted for questioning over accusations of sexual assault, which he has denied.

"Julian may die in jail over a nine-year persecution for revealing the truth of war crimes," Shipton said.

"It is beyond obscene."

His comments followed a warning from an independent UN rights expert last week that the treatment of Assange was putting his life "at risk".

"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.