WASHINGTON (AP) – The Bush administration, tightening the financial vise on Tehran, moved Tuesday to block the assets of a major Iranian bank suspected of helping spread weapons of mass destruction.

The action against Bank Sepah, Iran’s fifth-largest state-owned financial institution, means any banks accounts or other financial assets belonging to the bank found in the United States must be frozen. Americans also are prohibited from doing business with the financial institution.

“Bank Sepah is the financial linchpin of Iran’s missile procurement network and has actively assisted Iran’s pursuit of missiles capable of carrying weapons of mass destruction,” said Stuart Levey, Treasury’s under secretary for terrorism and financial intelligence.

The Treasury Department also added to its terror-blocking list Bank Sepah International Plc, a wholly owned subsidiary of Bank Sepah, and Ahmad Derakhshandeh, chairman and director of Bank Sepah.

The department alleges that Bank Sepah provides financial support and services to Iran’s Aerospace Industries Organization as well as to two Iranian missile firms – Shahid Hemmat Industries Group and the Shahid Bakeri Industries Group.America has previously accused all three of helping to spread weapons of mass destruction.

The government said that Bank Sepah is the “bank of choice” – at least since 2000 – for Iran’s Aerospace Industries Organization, a subsidiary of the Iranian Ministry of Defense and Armed Forces Logistics, that oversees all of Iran’s missile industries and is the overall coordinator of Iran’s missile program.

Bank Sepah “has provided a variety of critical financial services to Iran’s missile industry, arranging financing and processing dozens of multimillion dollar transactions” for Iran’s Aerospace Industries Organization and its affiliates, Levey said.

The government also alleged that the bank has facilitated business between the Aerospace Industries Organization and North Korea’s chief ballistic missile-related exporter, KOMID, which the government says has provided Iran with missile technology.

“The financial relationship between Iran and North Korea, as represented by the business handled by Bank Sepah, is of great concern to the United States,” Mr. Levey said.

Mr. Levey said Tuesday’s action applies to all branches of Bank Sepah, including those in Paris, Rome and Frankfurt, its wholly-owned subsidiary in London, as well as more than 290 branches based in Iran. The bank’s Rome branch, he said, conducted a “great deal” of the transactions the United States is concerned about.

The department has the power to act against Bank Sepah under an executive order issued by President Bush in June 2005.

It marked America’s latest move against Iran, a country America accuses of fostering terrorism and whose nuclear ambitions have drawn international rebuke.

Last year, the Treasury Department moved to cut off from the American financial system Bank Saderat – a big Iranian state-owned bank. The government said Iran used the bank to transfer money to terrorist groups, including Hezbollah.

Last month, the U.N. Security Council voted unanimously to impose economic sanctions on Iran. It did so because the country has refused to end a uranium enrichment program that America says is aimed at building nuclear weapons.