Baquer Namazi, 85, was permitted to leave Iran for medical treatment abroad, and his son Siamak, 50, was released from detention in Tehran, the United Nations said on Saturday.

"With the finalisation of negotiations between Iran and the United States to release the prisoners of both countries, $7 billion of Iran's blocked resources will be released," the state news agency IRNA said.

Billions of dollars in Iranian funds have been frozen in a number of countries -- notably China, South Korea and Japan -- since the US reimposed biting sanctions on the Islamic republic in 2018 after unilaterally withdrawing from Tehran's nuclear deal with world powers.

Tehran has accused Seoul of holding $7 billion of its funds "hostage", repeatedly calling on South Korean authorities to release it.

IRNA on Sunday said that "Washington is pursuing at the same time the release of its citizens detained in Tehran and the release of Iranian funds in South Korea".

The development comes as on-off talks have been underway since April 2021 to revive the 2015 deal that gave Iran much-needed sanctions relief in return for curbs on its nuclear programme.

Iran has repeatedly called for the lifting of sanctions, as well as guarantees that the United States will not again pull out of a revived deal.

Baquer Namazi is a former UNICEF official who was detained in February 2016 when he went to Iran to press for the release of his son Siamak, who had been arrested in October of the previous year.

Both were convicted of espionage in October 2016 and sentenced to 10 years in prison.

The father was released on medical leave in 2018 and had been serving his sentence under house arrest.