U.S. Secretary of State John Kerry is urging Iran to free former U.S. Marine Amir Hekmati from four years of "unjust detention."
August 29 will mark the fourth anniversary of Hekmati's imprisonment on what Kerry called "false espionage charges" while Hekmati was visiting relatives in the Islamic republic.
"We repeat our call on the Iranian government to release Amir on humanitarian grounds," Kerry said in a statement.
Kerry called on Tehran to release two other Americans, Christian pastor Saeed Abedini, who is serving an eight-year prison term, and Washington Post reporter Jason Rezaian, who has been imprisoned for more than a year.
Kerry also urged Tehran to "work cooperatively" with the United States to locate Robert Levinson, a former FBI agent who disappeared in Iran in 2007.