The return of $6bn (£4.8bn) of Iranian funds frozen in South Korea as part of a controversial prisoner swap has provoked a backlash from critics who say the money will be used to promote terrorism and harm America's interests.

The money was transferred as part of a deal that saw five dual US-Iranian citizens released from captivity.

Former US Defence Secretary Mark Esper told Fox News that the deal will "incentivise" Tehran to take more prisoners and force the US to pay more "ransom payments", which he said would probably be used to fund Iran's weapons and nuclear programme.

US officials have said the money is subject to oversight to ensure that Iran only uses it for humanitarian purposes.

Experts, however, have warned that there is little that the US can do to enforce those conditions - and Iran's president has said that the money will be used "wherever we need it".

The cash - which has been transferred to a restricted account in Qatar - was owed by South Korea to Tehran for oil bought before Trump administration sanctions in 2019 banned such transactions.

"It is Iran's money," state department spokesman Matthew Miller told reporters last week.

"So there are people who claim that we are giving them money, and we cannot give something that is not ours."

Mr Miller added that the US treasury department has "strict oversight" on the funds and that Washington "has the ability to police their use".

A senior administration official also told reporters that no funds will ever enter the country and will only be available for transactions for humanitarian goods with vetted third-party non-Iranian vendors.

The official said the US will take action to block the funds if Iran tries to divert them or use them for anything other than for the defined humanitarian purposes.

Those comments, however, stand in contrast to those made by Iranian President Ebrahim Raisi in an interview with NBC News last Tuesday.

Without offering more detail, Mr Raisi told the network that Iran has "authority" over how it spends the money and that "humanitarian means whatever the Iranian people need".

"The needs of the Iranian people will be decided and determined by the Iranian government," he said.

Many critics of the deal believe that Iran will probably use the money to support terrorist organisations fighting in the Middle East and produce more weapons to sell to Russia for its invasion of Ukraine.

Experts have warned that the US may have limited ability to oversee use of the funds.

"They will likely be able to see where it goes to start, but further downstream it will get murky," former deputy assistant secretary of defence for the Middle East, Mick Mulroy, told the BBC.

Alex Vatanka, the director of the Iran programme at the Washington DC-based Middle East Institute, said that there is little more the US and its allies can do other than "encourage" Iran to "stick to what they promised".

"It's impossible to measure where the term 'humanitarian' ends and something malign or sinister starts in terms of transactions," he said.

Democratic Senate Intelligence Committee chairman Mark Warner told CBS News on Sunday that he could not defend the deal. "Obviously money is fungible," he said, adding that he does not understand what "guardrails" have been placed on the cash by the US.

Mr Vatanka told BBC News that the recent release forms part of a larger trend of Iran using imprisoned dual nationals to secure the release of frozen funds.

Ali Safavi, a member of the Foreign Affairs Committee of the Paris-based National Council of Resistance of Iran, told the BBC that the group believes that the funds "will undoubtedly divert gains from hostage-taking to fuel regional proxy groups and destabilise the area further".

The council, also known as the NCRI, is considered to be the political arm of Mujahideen-e-Khalq (MEK), a dissident group that backs the overthrow of the Iranian government. It was once considered a terrorist organisation but was removed from the list in 2012.

In 2015, a similar - and controversial - agreement between Iran and the Obama administration saw funds released to Iran after four Americans, including a Washington Post journalist, were freed.

In the past, Iran has also detained citizens of a number of other countries including the UK, Canada, Australia, Germany and France.

One dual UK-Iranian national, Nazanin Zaghari-Ratcliffe, was released in March last year after the British government paid nearly £400m it owed Iran for failing to deliver tanks Iran bought in the 1970s.

"The script is predictable. They get picked up on charges of being spies or mobilising protests, Then, in return, they demand Iranians held in the US are released, or cash," Mr Vatanka said.

"As the Iranian economy becomes more and more desperate, we've seen Iran try to get its hands on cash, usually its own funds."

Iranian officials have acknowledged that the country's economy is plagued by high levels of inflation of between 40% and 50%, although experts believe the real figure may be higher.

In February, the Iranian currency fell to an all-time low of 600,000 rial to the US dollar, compared with 32,000 to the dollar at the time when a 2015 nuclear agreement was signed. The rial has since recovered much of the lost ground in recent months.

While Mr Vatanka said that Iran has repeatedly been successful in short-term "tactical" victories to retrieve sums of money, in the longer term the practice of arresting foreigners and dual nationals is probably "hurting the country in ways that will take years to show itself".

"This is going to hurt Iran so much. They have created a horrendous image," he added.

"They're going to not only put off foreign investors, but the millions of Iranian diaspora around the world that might want to invest in or visit Iran.

"They're turning off people in ways that nobody could have done for them."