The process of releasing US prisoners held in Iran will take up to two months, Iran's Foreign Ministry Spokesperson Nasser Kanaani said Monday during a press conference.

"A specific time frame has been announced by relevant authorities, and it will take a maximum of two months for this process to take place," Kanaani said.

Earlier this month, Tehran and Washington reached an agreement whereby five US citizens held hostage in Iran would be freed while $6bn of Iranian assets frozen in South Korea would be released. 

Iranian assets that had been frozen in South Korea were transferred to Switzerland's central bank last week for exchange and transfer to Iranian bank accounts in Qatar, South Korean media reported on Monday.

The deal foresees the funds remaining in Qatar to be spent on Iranian imports of food, medicine and other “non-sanctionable” goods. The US insists it will have oversight to make sure that the funds are not used for other purposes, however, Iran has said it has full control over the money.

Washington would also release some Iranians from US prisons, Iran said.

Iran allowed four detained US citizens to move into house arrest from Tehran's Evin prison, a lawyer for one said. A fifth was already under home confinement.

The deal has led to criticism in the United States by those who say the $6 billion is essentially a ransom paid for the hostages and it will only encourage Iran and other actors to detain Americans and demand money or concessions. Twenty-six Republican Senators have written to President Joe Biden demanding clarifications. Critics also argue that the money will provide Iran with the ability to spread its terror activities and other malign acts.