Here is a situation report from STRATFOR on how the US is pressuring South Korea to end economic ties with Iran:

The United States wants countries that buy Iranian petrochemicals to stop their purchases to pressure Tehran on its nuclear program, U.S. State Department special adviser Robert Einhorn said, AP reported Dec. 5. Speaking to reporters in Seoul, Einhorn said Washington and its allies want to send a message to Iran, and that Washington hopes South Korea will join the endeavor.  [STRATFOR]