WASHINGTON, Sept 11 (Reuters) - The United States waived sanctions to allow the transfer of $6 billion in Iranian funds from South Korea to Qatar, a step needed to carry out a previously announced U.S.-Iran prisoner swap, according to a U.S. document seen by Reuters on Monday.

The broad outlines of the U.S.-Iran deal under which five U.S. citizens detained by Iran would be allowed to leave in exchange for the transfer of the funds and the release of five Iranians held in the United States were made public on Aug. 10.

According to the State Department document seen by Reuters, Secretary of State Antony Blinken determined that waiving the sanctions was in the national security interests of the United States.

The document sent to U.S. congressional committees marks the first time the U.S. government has formally acknowledged it is releasing five Iranians detained in the United States as part of the agreement to secure the freedom of the five U.S. citizens.

"To facilitate their release, the United State has committed to release five Iranian nationals currently detained in the United States and to permit the transfer of approximately $6 billion in Iranian funds held in restricted accounts in the (Republic of Korea) to restricted accounts in Qatar, where the funds will be available only for humanitarian trade," it said.

The document said the transfer of funds would only provide "limited benefit to Iran" since the funds can only be used for humanitarian trade.

"Allowing these funds to be transferred from restricted Iranian accounts held in the (Republic of Korea) to accounts in Qatar for humanitarian trade is necessary to facilitate the release of these U.S. citizens," the document said.

White House spokesperson Adrienne Watson said in a statement late on Monday that Blinken, on Sept. 8, had undertaken "a procedural step in an ongoing process to ensure Iranian funds can move from one restricted account to another and remain restricted to humanitarian trade."

She said the administration had kept Congress informed from the start of the process.

"As we have said from the outset, what is being pursued here is an arrangement wherein we secure the release of 5 wrongfully held Americans. This remains a sensitive and ongoing process," she said. "While this is a step in the process, no individuals have been or will be released into U.S. custody this week."

The transfer of the $6 billion and the prisoner exchange could take place as early as next week, according to eight Iranian and other sources familiar with the negotiations.

The waiver applies to certain financial institutions that fall under the primary jurisdiction of Germany, Ireland, Qatar, South Korea and Switzerland to engage in transactions with the National Iranian Oil Company, the Central Bank of Iran and other Iranian financial institutions under U.S. sanctions, it added.