US Secretary of State John Kerry on Wednesday called for the release of two American dual citizens jailed in Iran, saying Washington was "deeply concerned" about their fate.

He also again requested information on an American who went missing in Iran in March 2007.

The United States is "deeply concerned about the fate of dual US citizens Amir Hekmati and Saeed Abedini," Kerry said in a statement.

Amir Mirzai Hekmati, a former US Marine, was sentenced to death in January 2012 on charges of espionage that Kerry said were "false."

Hekmati was shown on state television in mid-December 2011 saying in fluent Farsi and English that he was a Central Intelligence Agency operative sent to infiltrate Iran's intelligence ministry.

Hekmati's family in the United States said he had traveled to Iran to visit his grandmothers and insisted he was not a spy.

Abedini, a naturalized US citizen who converted from Islam to Christianity, was handed an eight-year prison sentence this January for his role in establishing underground Christian churches in Iran.

His wife Naghmeh, who lives in the US state of Idaho with their two children, had recently criticized US President Barack Obama for not speaking out on his behalf.

Kerry also called on Iran to help locate Robert Levinson, an ex-FBI agent who went missing while on a 2007 trip to the Islamic republic to probe cigarette counterfeiting in the region.

"His family has endured with courage and quiet dignity the pain of spending so many important milestones without him there. They shouldn't have to endure additional worry about his whereabouts and well-being," Kerry said.

Iran has insisted it has no new information about Levinson and that he is not in the country.