WikiLeaks founder Julian Assange was too ill to join an administrative hearing Monday, supporters said, as a court confirmed that his U.S. extradition proceedings will be delayed until September at the earliest.

Westminster Magistrates' Court said in a brief hearing via video that it was still seeking a venue to continue the extradition hearing, which was postponed last month because of concerns over the spread of coronavirus.

WikiLeaks said the failure to set a venue for a three-week extradition hearing in September compounds the "delays, irregularities and lack of due process" for Assange, who is being held on remand at London's Belmarsh prison.

The U.S. Justice Department has charged Australian citizen Assange, 48, with conspiring with former U.S. military intelligence analyst Chelsea Manning to leak a trove of classified material in 2010.

Free speech group Bridges for Media Freedom said Assange faces "an unprecedented (U.S.) Espionage Act prosecution for journalistic activity."

"Julian couldn't attend today's hearing via video link due to his health and on advice from his doctor," tweeted Assange's partner and former lawyer Stella Moris.

Rebecca Vincent of Paris-based Reporters Without Borders (RSF), said Assange faced "ongoing health concerns ... (and) the serious risk of exposure to COVID-19 in Belmarsh prison."

"This is another reason he should be immediately released," Vincent tweeted after following Monday's hearing.

British police arrested Assange in April 2019 at the Ecuadorian embassy, where he had taken refuge for seven years, for failing to surrender to an earlier warrant linked to Swedish charges that were eventually dropped.