US President Joe Biden hailed a prisoner swap with Iran as cause for celebration, but Republican criticism makes the deal politically risky ahead of the 2024 election.

The White House has defended the exchange, in which five "innocent Americans" flew out of Iran following the release of $6 billion in frozen Iranian funds. Five Iranians held in the US were also freed.

Biden said he would "continue to impose costs" on arch-foe Iran for its "provocative" actions and announced fresh sanctions against former Iranian president Mahmoud Ahmadinejad and the intelligence ministry.

"As we celebrate the return of these Americans, we also remember those who did not return," Biden said in a statement.

The White House meanwhile insisted that the "difficult" decision did not involve any US softening on Tehran, which Washington accuses of being the world's leading sponsor of terrorism.

"Sometimes in diplomacy you get what you can get," National Security Council spokesman John Kirby told CNN as the five Americans were flying home from Iran.

"I don't think we should look at this as some kind of confidence-building measure to a better relationship with Iran."

Kirby had last week denied that the release of the $6 billion was not a "ransom" and said that Iran was only allowed to use the money for food or humanitarian purposes.

A senior US administration official told reporters on Sunday they wanted to "dispel a few myths" about the deal.

"These are not taxpayer dollars. This is not a payment of any kind," the official said on condition of anonymity.

'Hostage swap'

But Republicans are already lashing out at Biden over the deal, as they eye unseating the Democrat in next year's presidential election.

"This is a hostage swap for $6 billion," Michael McCaul, Republican chair of the House Foreign Relations Committee, told Fox Business.

"And guess where it's going to go? It's going to go into terror proxy operations."

Criticism of the Iran deal comes at a tough time politically for Biden, who faces a Republican-led impeachment inquiry over his son Hunter's business affairs, while Hunter Biden has also been charged with a federal firearms offense.

Add a looming government shutdown and an autoworker strike that could damage the fragile economic gains of "Bidenomics," and Republicans have plenty of ammunition.

Contenders for the Republican primary nomination lined up to criticize the Iran deal when it first emerged last week on the anniversary of the 9/11 terrorist attacks on the United States.

Donald Trump, the likely Republican candidate next year, branded Biden an "incompetent fool."

"He had the audacity to announce this terrible deal today, September 11th," Trump, who tore up a nuclear deal with Iran while in office, wrote on social media.

Former Republican presidential candidate Mitt Romney warned that the deal could lead to more "kidnappings."

"The idea of basically paying to release, in this effect, a hostage is a terrible idea, the Utah senator said.

Iranian exile groups have given mixed reactions, with some criticizing the swap but others praising it.

And one person in particular thanking Biden: freed detainee Siamak Namazi.

"My heartfelt gratitude goes to President Biden and his administration, which had to make some incredibly difficult decisions," Namazi said in a statement.