TEHRAN, Iran >> The United States has proposed an exchange of Iranian prisoners in the U.S. for a former U.S. Marine currently in prison in Iran, the semiofficial ISNA news agency reported on Tuesday — a report which was immediately denied by Washington.

The report quoted Mahmoud Alizadeh Tabatabaei, the lawyer for Amir Hekmati, as saying Washington’s proposal was delivered to the Iranian government via the Swiss embassy — which represents Washington’s interests in Iran.

However State Department spokesman Jeff Rathke on Tuesday flatly denied the report of a prisoner-swap proposal and reiterated Washington’s call for Hekmati’s release on humanitarian grounds. Rathke also called for the release of pastor Saeed Abedini and Washington Post journalist Jason Rezaian, both currently detained in Iran, and for more Iranian help in finding former FBI agent Robert Levinson, who is believed to be missing in Iran.

“Those reports are not accurate,” Rathke said of the Hekmati report. “The U.S. government has not proposed a prisoner exchange for Mr. Hekmati. It’s not true.”

Hekmati, a dual U.S.-Iranian citizen born in Arizona, was arrested in August 2011 and sentenced to death for spying. Earlier this year, authorities overturned his death sentence and reduced his sentence to 10 years in prison.

The Obama administration in 2013 asked Iran to free Hekmati, as relations have recently thawed between Washington and moderate President Hassan Rouhani.

Hekmati, 31, told his family in Michigan by phone on Dec. 16 that he was beginning a hunger strike and dictated a letter asking President Obama not to forget him as negotiations proceed with Iran over its nuclear activities.

Washington and its international partners are hoping to clinch a deal with Iran that would set long-term limits on Iran’s enrichment of uranium. Iran says its nuclear program is solely meant for energy production and medical research purposes. However Tehran has been willing to discuss restricting its nuclear research in exchange for the lifting of crippling U.S. economic sanctions.

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Associated Press writer Bradley Klapper in Washington D.C. contributed to this report