Iran has released four Iranian Americans, one of them thought to be Washington Post reporter Jason Rezaian, as it anticipates the likely lifting of international sanctions imposed over its nuclear programme.
Rezaian was convicted of espionage in Iran last year.
The prisoners were not named but only four people with US citizenship are known to be in detention in Iran.
Iranian officials have been pressing for a prisoner exchange.
In addition to Rezaian, the Associated Press has named as US prisoners held in Iran
- Former Marine Amir Hekmati of Flint, Michigan
- Pastor Saeed Abedini of Boise, Idaho
- Siamak Namazi, a businessman and the son of a politician from the shah's era
The news agency adds that a former FBI agent, Robert Levinson, disappeared in Iran in 2007 while working for the CIA on an unapproved intelligence mission.
The Washington Post said in a report that there had been no official US confirmation of the release.
One Iranian news agency, Fars, named those released as Rezaian, Hekmati and Abedini. It did not give the fourth prisoner's name.