London -- 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 hearing on the U.S. request to have him extradited Thursday at Westminster Magistrates' Court.

WikiLeaks said it had "grave concerns" about Assange's health. The anti-secrecy group said he had been moved to the prison health ward, a move which they said spoke for itself.

The Reuters news agency quoted his lawyer, Gareth Peirce, as saying that his client was "in fact far from well."

Get Breaking News Delivered to Your Inbox

Assange 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.

Judge Emma Arbuthnot said a more substantive extradition hearing on the U.S. request, set for June 12, could be moved to a court next to Belmarsh prison for convenience.

Arbuthnot acknowledged the concerns for Assange's health, according to Reuters, saying in court: "He's not very well."

Sweden is also seeking Assange for questioning about an alleged rape. It will come down to the British judiciary authorities to decide which extradition request to honor, if either.

Reuters quoted the statement from WikiLeaks, saying that during Assange's term in prison, "his health has continued to deteriorate and he has dramatically lost weight."