Iran has denied reports that Washington offered to trade Iranian prisoners for a former U.S. Marine jailed since 2011.

Iranian Foreign Ministry spokeswoman Marzieh Afkham says there has been no formal request to trade U.S.-held prisoners for Amir Hekmati, 31, a dual U.S.-Iranian citizen born in Arizona, who was arrested in August 2011. The U.S. has also flatly denied the report and reiterated its call for Hekmati's release on humanitarian grounds.

Hekmati was convicted of spying for the CIA and sentenced to death. But earlier this year Iranian authorities reduced the sentence to 10 years in prison.

His lawyer, Mahmoud Alizadeh Tabatabaei, said Tuesday that Washington had delivered the proposal for the prisoner exchange through the Swiss embassy, which handles U.S. interests in Iran.