The Treasury Department says Iran is gaining access to the American financial system through the loophole of exchange houses and trading companies in countries not subject to U.S. sanctions.
The department says it has identified transactions that conceal Iranian companies and account holders, including people designated under U.S. sanctions law.
It said Thursday that U.S. banks don't need to close accounts held by exchange houses. But they should be diligent about where funds come from.
Washington is stepping up pressure on Iran over its nuclear program. The U.S. fears Iran may be trying to develop nuclear weapons; Iran says its program is for energy production.