The Washington Post says its journalist detained in Iran is in "immediate danger" as his health deteriorates and mistreatment of him intensifies.

The newspaper reports Wednesday it has submitted new information about Jason Rezaian to a U.N. working group on arbitrary detention. The newspaper says Thursday will mark 500 days since his arrest, with executive editor Martin Baron calling it the "grimmest" of milestones.

Rezaian has dual American and Iranian citizenship. Last month, Iranian state TV reported that he had been sentenced to an unspecified prison term following his conviction on charges that include espionage.

The newspaper says Rezaian did nothing wrong. Earlier this year, it appealed to the U.N. working group to intervene in the case.

Its new filing details the lack of openness and calls the proceedings "farcical."