Julian Assange is too ill to appear via a video link from a British prison, his lawyer says. (File)

WikiLeaks founder Julian Assange is too ill to appear via a video link from a British prison in a hearing on an extradition request from the United States, his lawyer said on Thursday.

The United States has requested the extradition of Assange, who was dragged from the Ecuadorean embassy in London on April 11. He faces a total of 18 US criminal counts with decades in prison if convicted.

"He's in fact far from well," Assange's lawyer, Gareth Peirce, said.

Judge Emma Arbuthnot said: "He's not very well."

WikiLeaks said it had grave concerns about Assange's health and that he had been moved to a health ward at Britain's Belmarsh prison. His health has deteriorated in prison and he has lost a lot of weight, WikiLeaks said.

The next hearing on the extradition request was set for June 12.