It's the latest in what has been decades of prisoner swaps between the two countries. The Americans include Siamak Namazi, held since 2015, and Morad Tahbaz and Emad Shargi, both detained in 2018.

The move comes amid the outlines of a deal after diplomacy between U.S. and Iran via mediators — but nothing is final.

Nazanin Zaghari-Ratcliffe, a charity worker, and Anousheh Ashouri, a businessman, have now left the country. A third prisoner was released on furlough. Other foreigners remain imprisoned in Iran.