Four US citizens imprisoned in Iran, including Washington Post journalist Jason Rezaian, are to be freed in a prisoner swap, Iran's semi-official FARS news agency reported. It comes with economic sanctions against Iran expected to be lifted in exchange for increased inspection of Iran's nuclear facilities.

All four prisoners are dual US-Iranian citizens, the news agency reported. Reziain has been held since July 2014, and was convicted of espionage last year. According to FARS, the country has freed Rezaian, Marine veteran Amir Hekmati and Christian pastor Saeed Abedini, who had been held on various charges. It did not identity the fourth person released.

The US has released six Iranian-US citizens who were held on charges relating to sanctions against Iran, reported the agency.

The news comes as leaders gathered in Vienna, with international sanctions expected to be lifted today in exchange for Iran curtailing its nuclear programme.