President Joe Biden expressed his appreciation Monday to South Korea and other countries for helping achieve the return of five American citizens who were detained in Iran.

Biden issued a statement celebrating their release under a rare swap deal with Tehran that reportedly saw around $6 billion in Iranian funds frozen in South Korean banks under US sanctions unlocked.

"I am grateful to our partners at home and abroad for their tireless efforts to help us achieve this outcome, including the Governments of Qatar, Oman, Switzerland, and South Korea," he said, noting the detainees endured "years of agony, uncertainty and suffering."

Biden gave special thanks to the Emir of Qatar, Sheikh Tamim bin Hamad, and the Sultan of Oman, Haitham bin Tariq, for helping facilitate the deal.

He also reminded US citizens of the "serious risks" of traveling to Iran, while stressing reuniting "wrongfully" detained Americans with their loved ones has been a priority for his administration.

In a separate statement, Secretary of State Antony Blinken also thanked South Korea.

"We also thank the Republic of Korea for its close coordination and partnership," he said, referring to the East Asian ally by its official name.

The Iranian funds in two South Korean banks -- the Industrial Bank of Korea and Woori Bank -- had been frozen under US sanctions that were reimposed after then US President Donald Trump withdrew from a landmark 2015 nuclear deal with Iran in 2018.

Seoul's foreign ministry issued a statement confirming the successful transfer of the Iranian funds to a third country under close cooperation between relevant nations.

It also expressed hope for the development of relations with South Korea and Iran following the transfer of the funds.

Tensions arose between South Korea and Iran as the Islamic republic had demanded the transfer of its frozen money. The demand posed a diplomatic challenge to Seoul, which has been striving to strengthen its treaty alliance with Washington and its economic partnership with Tehran. (Yonhap)