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Five Americans freed by Iran in a high-stakes prisoner swap landed back in the United States on Tuesday to a joyous reunion with family members.

The five arrived aboard an executive jet at an airfield in Fort Belvoir southwest of Washington.

Relatives waved US flags and hugged the freed prisoners as they disembarked from the aircraft, then posed for a group photograph, grinning widely.

"Welcome home," National Security Advisor Jake Sullivan posted on X, formerly Twitter.

The former prisoners, including one held for eight years, were part of a rare prisoner exchange between Washington and Tehran, a deal that included the unfreezing of $6 billion in fund, frozen by US ally South Korea.

The swap marked a slight thaw in relations between the two countries over a host of issues, including Iran's advances in its nuclear program, although some observers urged caution in viewing the release as a sign of change.

The prisoners arrived on a flight from the Gulf state of Qatar, which helped facilitate the exchange, negotiated over several months. They will receive a medical checkup in the Washington area.

The Biden administration has rejected criticism at home that it was paying "ransom," insisting the money will be used only for humanitarian purposes, with a threat to re-freeze the funds if not.

But Iran has insisted it has full access.

The money "cruelly blocked until now and currently in the possession of the Islamic republic belongs to the people (of Iran) and we will use them to meet the people's needs," President Ebrahim Raisi said in New York.

'Not naive'

Both Raisi and US President Joe Biden will be among world leaders gathered in New York Tuesday for the United Nations General Assembly. They are not scheduled to meet.

One of the freed prisoners praised Biden for ignoring the political backlash and taking the "incredibly difficult decisions" that freed them.

"Thank you, President Biden, for ultimately putting the lives of American citizens above politics," Siamak Namazi, a businessman held by Iran since 2015, said in a statement.

Other freed prisoners include wildlife conservationist Morad Tahbaz and venture capitalist Emad Sharqi, both of whom were held at the notorious Evin Prison but placed under house arrest last month.

Two other US prisoners involved in the swap have not been publicly identified. All are Iranian-Americans.

The five Iranian prisoners released by the United States were convicted or charged with nonviolent crimes, with one already set to be released soon, officials said.

Some observers viewed the prisoner exchange cautiously.

"We are not naive," European Council President Charles Michel told journalists Monday at the United Nations.

"We can also observe the very brutal repression" in Iran, he said, including "the use of kidnappings by the Iranian authorities to put pressure on some governments including in the EU."

"We do not underestimate the level of tensions and difficulties."