WikiLeaks founder Julian Assange has missed a court session, apparently due to health problems.

Assange had been expected to appear from prison via video link at a brief extradition hearing Thursday at Westminster Magistrates' Court.

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He is in Belmarsh prison serving time for jumping bail in Britain while fighting extradition to the United States, which accuses him of violating the Espionage Act by publishing secret documents containing the names of confidential military and diplomatic sources.

Sweden also wants him for questioning about an alleged rape.

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Judge Emma Arbuthnot said a more substantive extradition hearing set for June 12 may be moved to a court next to Belmarsh prison for convenience.

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WikiLeaks says it has "grave concerns" about Assange's health. The anti-secrecy group says he has been moved to the prison health ward.

"During the seven weeks in Belmarsh his health has continued to deteriorate and he has dramatically lost weight," a spokesman said.