Hours after Iranian officials said Rezaian, 39, was freed from Tehran's notorious Evin Prison after 18 months of captivity, and after a planned ceremony on the nuclear accord in Vienna was repeatedly postponed, the Post journalist and the other Americans remained in Tehran, waiting to be flown out of the country aboard a Swiss plane.

There was no immediate explanation for the delay.

But the head of the International Atomic Energy Agency then announced the release of a report Saturday "confirming that Iran has completed the necessary preparatory steps to start the implementation" of the nuclear deal, a move that apparently paves the way for the exchange to go ahead.

Secretary of State John F. Kerry subsequently signed documents at the site of the nuclear negotiations with Iran.