US President Barack Obama has signed tough new sanctions against Iran into law.
A new defence law cuts foreign companies that do business with Iran's central bank from the US financial system.
The president signed the law in Hawaii, where he is on holiday. The BBC reports there is heightened tension over the Iranian nuclear programme.
Western powers say Tehran is seeking to acquire a nuclear weapons capability, but Iran insists the programme is purely for domestic energy purposes.
Iran is currently holding 10 days of naval exercises in the Gulf.