The United States believes an understanding on Iran eventually releasing five US citizens remains on track, White House National Security Adviser Jake Sullivan has said but declined to offer any timeline.

"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 on Tuesday in a conference call.

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 the country.

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 programme to Tehran's support for regional militias.

The Iranian Americans who were allowed to leave Iran's Evin prison on August 10 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.