An understanding with Iran to eventually release five Americans remains on track, White House National Security Adviser Jake Sullivan said Tuesday but declined to offer any timeline.

Iran on August 10 released four imprisoned US citizens into house arrest, where they joined a fifth already under home confinement, in the first step of a deal under which $6 billion in Iranian funds in South Korea would be unfrozen and the five would eventually be allowed to leave.

"We believe that things are proceeding according to the understanding that we've reached with Iran. I don't have an exact timetable for you because there's steps that need to yet unfold. But we believe that that remains on track," Sullivan told reporters in a conference call.

South Korean media reported Monday the money has already been transferred to the Swiss central bank that would convert the funds to euros and send it to Iranian bank accounts in Qatar. Iran’s foreign ministry said Monday that it would take two months for the hostages to be allowed to leave.

Allowing the five to leave Iran, which could take weeks, would remove a major irritant between Washington and Tehran, which remain at odds on issues from the Iranian nuclear program to Tehran's support for regional Shi'ite militias.

The Iranian Americans who were allowed to leave Iran's Evin prison included businessmen Siamak Namazi, 51, and Emad Shargi, 58, as well as environmentalist Morad Tahbaz, 67, who also has British nationality. The identity of the fourth US citizen who left the prison has not been made public, nor has that of the fifth who was already under house arrest.