Four prisoners with dual Iranian-US nationality have been released by Tehran, with a Washington Post reporter believed to be among them.
The inmates are yet to be named - but an anonymous Iranian official has confirmed to the AP news agency that Jason Rezaian, a journalist convicted of espionage last year, was freed.
It is also understood that seven Iranian nationals who are currently serving sentences in American jails will be released as part of the prisoner swap agreement.
The exchange came as Iran prepares to seal a historic nuclear deal with the US, the EU and the International Atomic Energy Agency - which was thrashed out in Vienna last summer following extensive talks.
All parties involved have now gathered again in the Austrian capital following work by the IAEA to check that Iran has complied with their commitments to curtail their nuclear programme.
It means sanctions that have crippled the Iranian economy are now expected to be lifted.
Iran's foreign minister Javad Zarif said in Vienna: "We expect to have the implementation day today.
"I believe it is not just an important day for economic activity in Iran.
"It is going to open the possibilities, vast possibilities in Iran for economic engagement and we have all seen that the international business community is very much interested in getting engaged in Iran to take advantage of it.
"Today is the day when we prove to the world that threats, sanctions, intimidation, pressure don't work."
US Secretary of State John Kerry and EU foreign policy chief Federica Mogherini are among the representatives back in Vienna ahead of the announcements that are expected to be confirmed later.
Lifting sanctions would allow Iran to immediately recoup some $100bn (£70bn) in assets frozen overseas.
However, it is the new business opportunities in oil, trade and finance that could revolutionise the struggling Iranian economy.
The agreement puts Iran's various nuclear activities under IAEA watch for up to 15 years, with an option to reimpose sanctions should Tehran break its commitments.
