(Bloomberg) -- Iran moved four US citizens from prison to house arrest in the first step of an emerging deal in which the US is expected to unfreeze $6 billion in assets to Tehran and free several jailed Iranians.

The American prisoners include Siamak Namazi, who has been held in Tehran’s Evin prison since October 2015, along with three others, while a fifth American may also be freed. Iran’s state-run media said four US citizens will be released in exchange for four or more unidentified Iranians detained in the US and only after the $6 billion in funds that are blocked in South Korea are transferred into an account in Qatar.

US officials declined to describe the terms of the deal, saying that revealing more risked upsetting a delicate process that could still fall apart. Yet their reticence also appeared intended to head off the inevitable criticism from Republicans and others that President Joe Biden was giving up too much to the regime in Tehran.

Earlier: US, Iran Inch Toward Understanding Aimed at Easing Tensions 

The US State Department has spoken to the Americans, Secretary of State Antony Blinken said, describing the transfer as a “positive step.” He added that he wouldn’t “get into any of the details about what we’re doing or engaged on because I don’t want to jeopardize the completion of this process.”

But Iran’s semi-official Tasnim news agency gave more details, saying the Americans would be freed only once Iran gets the $6 billion. It said the Iranians to be released were being held by the US for sanctions violations and described it as a direct quid pro quo. 

“Iran has received the necessary guarantee for America’s adherence to its obligations,” Iran’s Deputy Foreign Minister Ali Bagheri Kani said in a social media post. “The release of several Iranians who were illegally detained in America is in this context.”

A person briefed on the agreement, who asked not to be identified discussing terms that haven’t been disclosed, said the US requested on Thursday that the money be transferred from South Korea to Switzerland and then to Qatar, which will oversee how it’s spent. The prisoners will be swapped only after the transfer is complete, a process that should take 30 to 45 days, the person said.

South Korea’s Ministry of Foreign Affairs declined to confirm the transfer, but said the government had been “in close consultation with the US, Iran and other relevant nations to resolve the issue of frozen funds.”

The country’s refiners use two banks for transactions with Iran. Asked about the matter, a spokesperson for the Industrial Bank of Korea told Bloomberg it would deal with the matter in line with government policy, while Woori Bank was not immediately available for comment.

Secretive Diplomacy

The releases were the latest step in tentative and secretive diplomacy that’s seen the US and Iran inch toward an informal understanding under which Tehran would free the Americans and potentially impose limits on its nuclear program.

The Swiss government, which represents US interests in Tehran in the absence of formal diplomatic relations, served as an intermediary, according to the person, who also credited Qatar and Oman with helping facilitate the process.

Those negotiations — conducted through intermediaries in Oman and elsewhere — led to an initial understanding for Iran to free prisoners while the US would allow for the release of the more than $6 billion in frozen funds that South Korea owes to Iran for oil purchases, Bloomberg News reported previously. Iran said it would be free to decide how to spend the funds, unlike previous similar waivers where the US has restricted purchases to humanitarian goods like food and medicine.

Iran’s state-run Islamic Republic News Agency said Tehran is gaining access to more than $10 billion in funds that had been frozen — including a “significant amount” in the Trade Bank of Iraq that is being transferred to Qatar’s central bank via a Swiss bank, the report said.

As far back as March, Iran’s foreign affairs minister Hossein Amirabdollahian said Iran and the US were preparing to carry out a prisoner exchange. Then-State Department spokesman Ned Price called that claim a “cruel lie.”

All along, US officials have been cagey about the terms of the deal, wary of acknowledging talks with a regime that has ramped up human rights abuses and continues to supply weapons and other material to Russia for its invasion of Ukraine. At the same time, Biden and his top officials say former President Donald Trump made a grave error in 2018 when he backed out of deal, signed in 2015, that imposed limits on Iran’s nuclear program.

Trump’s team, led by Secretary of State Michael Pompeo, launched a “maximum pressure” campaign designed to crush the country’s economy and its nuclear ambitions. In the years since, Iran has pressed ahead, enriching uranium close to the levels needed for nuclear weapons. The country denies it wants a nuclear bomb.

Any deal will raise criticism from Biden’s opponents, who hammered the Obama administration over the terms of the previous nuclear deal. In 2016, the US airlifted $400 million to Iran as it released four detained Americans. President Barack Obama said the payment wasn’t a ransom and was from a decades-old dispute.

Read More: Iran, US to Exchange Prisoners ‘Soon’, Foreign Minister Says 

“You get what you pay for, as the saying goes,” Robert Greenway, a senior National Security Council director under Trump, said of the latest developments in a social media post on Thursday. “While it’s encouraging to see the unjustly detained return home to families, rewarding horrific behavior doesn’t work any better with rogue regimes than it does with pets.”

Iran has made tentative steps toward trying to ease tensions in the region with an agreement to normalize relations with Saudi Arabia and further cooperation with the International Atomic Energy Agency. With a new nuclear deal politically unpalatable in the US, Western diplomats have been pushing Iran to voluntarily limit its uranium-enrichment levels and bolster its cooperation with international monitors in return for allowances to ship more crude.

On Thursday, National Security Council spokeswoman Adrienne Watson, called the releases from prison an encouraging step but said the Americans “should have never been detained in the first place.” State Department spokesman Matthew Miller said they “must be allowed to depart Iran and reunite with their loved ones as soon as possible.”

--With assistance from Annmarie Hordern, Eric Martin, Kateryna Kadabashy, Daedo Kim and Sangmi Cha.

(Updates with comments from South Korea in eighth paragraph.)

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