Washington suspects the program may aim to develop nuclear weapons – an ambition Tehran denies – that could threaten Israel or US Gulf Arab allies.
US Secretary of State Antony Blinken left the door open to diplomacy on the nuclear file, which he described as “perhaps the number one issue of concern,” but suggested nothing was imminent.
“In this moment, we’re not engaged on that, but we’ll see in the future if there are opportunities,” he told reporters in New York.
US analysts voiced scepticism that progress was likely soon on the nuclear or other issues.
“The prisoner swap does likely pave the way for additional diplomacy around the nuclear program this fall, although the prospect for actually reaching a deal is very remote,” said Henry Rome of the Washington Institute for Near East Policy.
“Removing an irritant is different from adding a salve,” said Jon Alterman of the Centre of Strategic and International Studies.
In a sign Biden, a Democrat, wishes to appear tough on Iran and perhaps blunt Republican criticism, he announced fresh US sanctions on Iran’s former president Mahmoud Ahmadinejad and its intelligence ministry for “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 United States and Iran do not diplomatic relations, helped with the transfer of funds from South Korea to Qatar.
A plane sent by Qatar flew the five US citizens and two of their relatives out of Tehran after both sides got confirmation the $US6 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 aides argue the money belongs to Iran and is being transferred from restricted South Korean accounts to restricted Qatari accounts, where it can only be spent on food, medicine and other humanitarian items with US oversight.
Earlier, two of the five Iranians landed in Qatar, a US official said. Three have opted not to return to Iran.
One of the five freed Americans had been held for about eight years on charges Washington rejected as baseless.
The deal, after months of talks in Qatar, removes a major irritant between the US, which brands Tehran a sponsor of terrorism, and Iran, which calls Washington 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 British 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.
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