SEOUL (Reuters) - Efforts are under way to transfer Iran's funds that had been frozen in South Korea, foreign minister Park Jin said, after Teheran reached a deal with the United States to release American citizens in return for freeing Iranian assets.

Park told his Iranian counterpart, Hossein Amir-Abdollahian, in a phone conversation on Monday that involved countries were in close communications to resolve the issue, Seoul's foreign ministry said in a statement on Tuesday.

Park and the Iranian foreign minister agreed to expand cooperation between the two countries, it said.

The United States and Iran reached an agreement last month for the release of five U.S. citizens detained in Iran while $6 billion of Iranian assets in South Korea would be unfrozen.

The assets that had been frozen in South Korea were transferred to Switzerland's central bank in August for exchange and transfer to Iran, South Korean media has reported.

(Reporting by Soo-hyang Choi; Editing by Shri Navaratnam)