Five US detainees have flown out of Iran in a swap for five Iranians held in the US under a rare deal brokered by Qatar between the arch enemies that also unfroze $US6 billion ($A9.3 billion) of Tehran's funds.
A Qatari plane took off from Tehran carrying the five with two of their relatives, shortly after the US and Iran received confirmation that the funds had been transferred to accounts in Doha, a source briefed on the matter told Reuters.
Iran's Press TV said two of the five Iranians to be released in the exchange had landed in Doha. There was no independent confirmation, though Iranian officials had said three of those to be freed by the US were not returning to Iran.
The funds' release triggered an exchange sequence agreed after months of talks between the United States and Iran, who are at odds over Tehran's nuclear ambitions and other issues.
The five Americans with dual nationality are due to fly to Doha and then on to the US.
"They are in good health," an Iranian official briefed on the process said.
Iran's Foreign ministry spokesperson Nasser Kanaani said two of the Iranians being released would return to Iran while two would stay in the US at their request. One detainee would join his family in a third country, he added.
Kanaani said the funds, blocked in South Korea after US sanctions on Iran were hardened in 2018, would be available to Tehran on Monday. Under the deal, Qatar will ensure the cash is spent on humanitarian goods and not items under US sanctions.
The deal will remove a major irritant between the US, which brands Tehran a state sponsor of terrorism, and Iran, which calls Washington the "Great Satan".
But they remain deeply divided on other issues ranging from Iran's nuclear program and its influence around the region to US sanctions and America's military presence in the Gulf.
Qatar, a tiny but hugely wealthy Gulf Arab energy producer, has sought to raise its global profile, hosting the soccer World Cup last year and carving out a role in international diplomacy. The Sunni Muslim nation hosts a big US military base but has also forged close ties with Shi'ite Muslim Iran.
Doha hosted at least eight rounds of talks with Iranian and US negotiators sitting in separate hotels, speaking via shuttle diplomacy, a source previously told Reuters.
The transfer of Iran's funds under the agreement has drawn criticism from US Republicans who say President Joe Biden, a Democrat, is in effect paying a ransom for US citizens.
The White House has defended the deal.
The US dual citizens to be released include Siamak Namazi, 51, and Emad Sharqi, 59, both businessmen, and Morad Tahbaz, 67, an environmentalist who also holds British nationality. They were released from prison and put under house arrest last month.
A fourth US citizen was also released into house arrest, while a fifth was already under house arrest. Their identities have not been disclosed.
Iranian officials have named the five Iranians to be released by the US as Mehrdad Moin-Ansari, Kambiz Attar-Kashani, Reza Sarhangpour-Kafrani, Amin Hassanzadeh and Kaveh Afrasiabi. Two Iranian officials previously said that Afrasiabi would remain in the United States but had not mentioned others.