A plane carrying five US citizens freed by Iran has left Doha as part of a prisoner swap for five Iranians held in the United States amid the transfer of billions of dollars in Iranian funds, marking a rare deal between the long-time antagonists.
"Today, five innocent Americans who were imprisoned in Iran are finally coming home," US President Joe Biden said in a statement shortly before the US detainees descended the stairs of a Qatari jet to be embraced by US diplomats.
The White House confirmed a plane carrying the five, along with two US family members who left Tehran with them earlier, was en route to the United States, where they were to be offered medical treatment by the US military as they adjust to freedom.
Separately, Iran's state-owned Press TV said the five Iranians held by the United States and charged with committing crimes had been freed, an apparent reference to their being granted clemency, and that two of them had landed in Tehran.
The other three are not expected to return to Iran.
"This was purely a humanitarian action," Iranian President Ebrahim Raisi said after arriving in New York for the annual United Nations General Assembly.
"It can certainly be a step based upon which in the future other humanitarian actions can be taken."
It was unclear whether the exchange might bring progress on the many issues that divide the two governments, including Iran's nuclear program, its support for regional Shi'ite militias, the presence of US troops in the Gulf and US sanctions on Iran.
In a sign Biden, a Democrat, wishes to keep a tough front toward Iran and perhaps blunt Republican criticism, he announced US sanctions on Iran's former Iranian president Mahmoud Ahmadinejad and its intelligence ministry "for their involvement in wrongful detentions".
"We will continue to impose costs on Iran for their provocative actions in the region," he said in the statement, in which he thanked the governments of Qatar, Oman, Switzerland and South Korea for their assistance in securing the releases.
Qatar mediated indirect US-Iran talks on the detainees while Switzerland, which represents US interests in Tehran because the US and Iran do not diplomatic relations, helped with the transfer of funds from South Korea to Qatar.
A plane sent by mediator Qatar flew the five US citizens and two of their relatives out of Tehran after both sides got confirmation $US6 billion ($A9.3 billion) was transferred from South Korea to Qatari accounts, a source briefed on the matter told Reuters.
A prominent Republican, House Foreign Affairs Committee Chairman Michael McCaul, said the transfer of the $US6 billion would likely lead to more detentions of US citizens by Iran.
"I am very concerned that this $US6 billion hostage deal incentivises future hostage-taking," McCaul said in an emailed statement.
"There is no question this deal will free up funds for Iran's malign activities."
Biden's aides argue the money belongs to Iran and is being transferred from South Korea to Qatari accounts, where it can only be spent on food, medicine and other humanitarian items with US oversight.
Earlier, two of the five Iranian citizens landed in Qatar, a US official said.
Three have opted not to return to Iran.
One of the five freed US citizens had been held for about eight years on charges the US rejected as baseless.
The deal, after months of talks in Qatar, removes a major irritant between the US, which brands Iran a sponsor of terrorism, and Iran, which calls the US the "Great Satan".
The US dual citizens released include Siamak Namazi, 51, and Emad Sharqi, 59, both businessmen, and Morad Tahbaz, 67, an environmentalist who also holds UK nationality.
The identities of the fourth and fifth US citizens were not disclosed at their request given their desire for privacy.
Iranian officials have named the five Iranians released by the US as Mehrdad Moin-Ansari, Kambiz Attar-Kashani, Reza Sarhangpour-Kafrani, Amin Hassanzadeh and Kaveh Afrasiabi.
Australian Associated Press
