The United States has blacklisted one of Iran's largest banks, accusing it of a key role in financing the development of weapons of mass destruction.
Iran insists that it nuclear programme aims to provide energy
A US treasury official said state-owned Bank Sepah was the "financial lynchpin" in Iran's missile procurement network.
The move freezes its assets in the US and bars any US companies or citizens from doing business with the bank.
The US says Iran is working to acquire nuclear weapons. Iran says its nuclear programme is for peaceful purposes.
"Bank Sepah is the financial lynchpin of Iran's missile procurement network and has actively assisted Iran's pursuit of missiles capable of carrying weapons of mass destruction," Treasury Undersecretary Stuart Levey said.
He said the bank had facilitated Iran's international purchases of material for its missile programme.
Bank Sepah, he said, had been the conduit for a deal with Komid, a North Korean organisation which the US says gave missile technology to Iran.
The blacklisting extends to Bank Sepah's UK-based subsidiary, Bank Sepah International Plc.
In December, the United Nations Security Council unanimously voted to impose sanctions against Iran over its failure to halt uranium enrichment.
The sanctions ban the supply of nuclear-related technology and materials and impose an asset freeze on key individuals and companies.