Iran releases Jason Rezaian and three other American prisoners
The report did not identify the prisoners and the person in Iran was speaking on condition of anonymity since he was not authorised to speak publicly to the media.
Rezaian is a dual Iran-US citizen who was convicted of espionage in a closed-door trial in 2015.
Washington Post spokeswoman Kris Coratti told the AP that she could not confirm any change in Rezaian's case.
A report by the semi-official ISNA news agency quoted a statement from the Tehran prosecutor's office as saying the inmates were freed "within the framework of exchanging prisoners." It did not elaborate.
US officials would not immediately confirm the announcement, but they had indicated a prisoner deal would be separate from Saturday's expected "implementation" of the landmark nuclear agreement.
Iranian Foreign Minister Mohammad Javad Zarif, in Vienna to meet US Secretary of State John Kerry, spoke cryptically of a possible negotiation.
The family of one of the US prisoners received unofficial word from Iran that their relative was being released Saturday, according to a person close to that family who spoke on condition the family not be identified.
Iran was seeking a number of detained Iranians in exchange for the Americans.