Julian Assange's mother has accused the US and UK governments of "slowly, cruelly and unlawfully" killing the WikiLeaks founder because he revealed war crimes and corruption.

"My son Julian Assange is being slowly, cruelly & unlawfully assassinated by the US and UK Governments, for multi-award winning journalism revealing war crimes & corruption!" she wrote.

She also tweeted a link to a report from a United Nations expert who called for a stop to the "collective persecution" of the whistleblower.

Assange is currently incarcerated in Belmarsh Prison after being arrested at the Ecuadorian Embassy in London in April. His visitors have repeatedly raised concerns about his health. As recently as last Wednesday, journalist John Pilger said his condition was "deteriorating" and that he was being treated "worse than a murderer."