Five Americans detained for years in Iran walked off a plane and into freedom Monday, most arm-in-arm, as part of a politically risky deal that saw U.S. President Joe Biden agree to the release of nearly $6 billion in frozen Iranian assets owed by a third country, South Korea.

"Today, five innocent Americans who were imprisoned in Iran are finally coming home," Biden said in a statement released as the plane carrying the group from Tehran landed in Doha, Qatar. A plane carrying the Americans home to the United States was due to land Monday night.

Iran's President Ebrahim Raisi, on hand for the United Nations General Assembly in New York, suggested Monday's exchange could be "a step in the direction of a humanitarian action between us and America."

"It can definitely help in building trust," Raisi told journalists.

However, tensions are almost certain to remain high between the U.S. and Iran, which are locked in disputes over Tehran's nuclear program and other matters. Iran says the program is peaceful, but it now enriches uranium closer than ever to weapons-grade levels.

The prisoner release unfolded amid a major American military buildup in the Persian Gulf, with the possibility of U.S. troops boarding and guarding commercial ships in the Strait of Hormuz, through which 20 percent of all oil shipments pass.

After the plane slowed to a stop in Doha, three of the prisoners, Siamak Namazi, Emad Sharghi and Morad Tahbaz, walked down the stairs. The United States did not immediately identify the other two freed Americans, all of whom were released in exchange for five Iranians in U.S. custody and for the deal over the frozen Iranian assets. The Biden administration said the five freed Iranians pose no threat to U.S. national security.

Iranian Foreign Ministry spokesman Nasser Kanaani said two of the Iranian prisoners will stay in the U.S. Meanwhile, Nour News, a website believed to be close to Iran's security apparatus, said two of the Iranian prisoners were in Doha for the swap.

Nour News identified the two released Iranians in Doha as Mehrdad Ansari, an Iranian sentenced by the U.S. to 63 months in prison in 2021 for obtaining equipment that could be used in missiles, electronic warfare, nuclear weapons and other military gear, and Reza Sarhangpour Kafrani, an Iranian charged in 2021 over allegedly unlawfully exporting laboratory equipment to Iran.

The $5.9 billion in cash released to Iran represents money South Korea owed Iran, but had not yet paid, for oil purchased before the U.S. imposed sanctions on such transactions in 2019.

The U.S. maintains that, once in Qatar, the money will be held in restricted accounts to be used only for humanitarian goods, such as medicine and food. Those transactions are currently allowed under American sanctions targeting the Islamic Republic over its advancing nuclear program.

Iranian government officials have largely concurred, though some hard-liners have insisted, without evidence, that there would be no restrictions on how Tehran spends the money.

The planned exchange comes ahead of the convening of world leaders at the UN General Assembly this week in New York, where Raisi will speak.

The deal has already opened Biden to fresh criticism from Republicans and others who say the administration is helping boost the Iranian economy at a time when Iran poses a growing threat to American troops and Mideast allies. That could have implications in his re-election campaign.

Former President Donald Trump, currently the lead Republican challenger in the polls against Biden's 2024 re-election bid, called it an "absolutely ridiculous" deal on the Truth Social social media site. Senate Minority Leader Mitch McConnell accused Biden of "rewarding and incentivizing Tehran's bad behavior."

In a statement, Biden urged Americans not to travel to Iran and demanded more information on what happened to Bob Levinson, an American who went missing years ago. The Biden administration also announced fresh sanctions on former Iranian President Mahmoud Ahmadinejad and the Iranian Ministry of Intelligence.

Iran and the U.S. have a history of prisoner swaps dating back to the 1979 U.S. Embassy takeover and hostage crisis following the Islamic Revolution. Their most recent major exchange happened in 2016, when Iran came to a deal with world powers to restrict its nuclear program in return for easing sanctions.

The West accuses Iran of using foreign prisoners, including those with dual nationality, as bargaining chips, an allegation Tehran rejects.

Negotiations over a major prisoner swap faltered after then-president Trump unilaterally withdrew America from the nuclear deal in 2018. From the following year on, a series of attacks and ship seizures attributed to Iran have raised tensions.

(With input from AP)