Fannon said the decision was made after a year of consultations with South Korea and other importing nations. He also said all importers were treated equally.

"I think the alliance between the United States and South Korea is certainly an enduring one. That continues," Fannon said. "I can say the ongoing discussions to help South Korea, those conversations have been going on for a year, and they're going to continue to go on, because the alliance is critically important on so many areas beyond this one."

South Korea relies heavily on Iranian condensate to produce petrochemical products, which are a leading export item, along with semiconductors.

Fannon stressed that there are increasing volumes of oil and condensates on the market to satisfy global demands. As for the specific alternative sources, that would be up to South Korean companies to decide, he said.

The import ban is among the sanctions Washington imposed on Tehran last year after President Donald Trump withdrew the US from the 2015 nuclear deal with Iran.

This month the US also designated Iran's Islamic Revolutionary Guard Corps as a foreign terrorist organization. It marked the first time the US has declared an entity of a foreign government to be a terrorist organization.

"We will continue to apply maximum pressure on the Iranian regime until its leaders change their destructive behavior, respect the rights of the Iranian people, and return to the negotiating table," US Secretary of State Mike Pompeo said in a statement. (Yonhap)