Five U.S. detainees flew out of Iran on September 18 as part of a swap between the United States and Iran after months of talks to clinch a deal.
A Qatari plane took off from Tehran carrying the five with two of their relatives, news agencies reported, hours after an Iranian Foreign Ministry spokesperson said the prisoner exchange would occur shortly.
Some $6 billion of Iranian assets once frozen in South Korea are now in Qatar, a key element for the prisoner exchange, added ministry spokesman Nasser Kanaani in comments during a news conference aired on state television.
The exchange comes amid a major U.S. military buildup in the Persian Gulf. The deal has already opened President Joe Biden to fresh criticism from Republicans and others who says his administration is helping boost the Iranian economy at a time when Iran is posing a growing threat to U.S. troops and Mideast allies.
According to the deal, the funds will be kept in accounts in Qatar, a U.S. ally on the Arabian Peninsula and home to a major American military installation. Those funds would be allowed for so-called humanitarian spending, such as food and medicine, already allowed under the sanctions, the United States has said.
Iranian officials have identified five individuals in U.S. custody whom Tehran would like handed over as part the deal.
They include three Iranians -- Mehrdad Ansari, Reza Sarhangpour Kafrani, and Kambiz Attar Kashani -- charged with illegally obtaining advanced or potential dual-use technology thought to be bound for Iran that has been under tightly reimposed U.S. sanctions since 2018.
Two others -- Kaveh Lotfolah Afrasiabi and Amin Hasanzadeh -- were jailed for failing to register as foreign agents and stealing engineering plans on behalf of Iran, respectively.
"Out of the five Iranian citizens in America, two will return to Iran, two will stay in America at their own request, and one person will go to a third country at their request," Kanaani said. He did not identify which prisoners would return to Iran and which would not.
The American prisoners had included Siamak Namazi, who was detained in 2015 and was later sentenced to 10 years in prison on internationally criticized spying charges; Emad Sharghi, a venture capitalist sentenced to 10 years; and Morad Tahbaz, a British-American conservationist of Iranian descent who was arrested in 2018 and also received a 10-year sentence.
The fourth and fifth prisoners were not identified.
Iran has been accused of taking foreign nationals hostage under the guise of breaking the law to use as bargaining chips. Iranian security forces have taken some 40 foreign nationals into custody during a current wave of unrest, often without revealing any charges.
Iran has been isolated and hit with tightened economic and diplomatic sanctions since then-President Donald Trump unilaterally withdrew in 2018 from a 3-year-old deal between world powers and Iran to curb Tehran's nuclear program in exchange for relief from previous measures aimed at stopping the country from developing its atomic capabilities.
Aside from the diplomatic and economic fallout, observers since then have attributed a series of ship seizures and attacks in the crucial Strait of Hormuz region to Tehran.
The Pentagon is said to be weighing a plan to put U.S. troops aboard commercial ships in the region, which is a conduit for around one-fifth of all global oil shipments.
Tehran has also cooperated with Russia in the Middle East in addition to supplying Moscow with crucial attack drones to further the Kremlin's war plans in Ukraine.