Samsung Total Petrochemicals, a joint venture between South Korea's Samsung Group and French energy giant Total, bought Iranian condensate because thin profit margins in plastics production make inexpensive fuel from the Islamic Republic hard to resist, people familiar with the deal said last week.
As the United States and the European Union placed new sanctions on purchases of oil from Iran last year, South Korea slashed crude purchases from Iran 36 percent to about 153,000 barrels per day in 2012.
The sanctions aim to slow Tehran's nuclear program, which Western powers believe is aimed at developing weapons. Iran says the program is for civilian purposes.
Under the U.S. sanctions law, banks in countries where Iranian oil is bought can be cut off from the U.S. financial system unless the purchases are reduced.
The State Department on Monday said South Korea is still on the same page with the United States regarding Iran.
"We continue to engage in close consultations with the Korean government on U.S. sanctions and share the objective of maintaining pressure on Iran to comply with its international obligations," said spokesman John Finn.
He said South Korea has significantly reduced its imports of Iranian oil and that "month-to-month variability in crude oil purchases is not unusual."
Finn declined to comment on whether U.S. officials had talked with the South Korean government since the deal came to light last week.
Washington has given all of Iran's major crude buyers six-month exceptions to the sanctions in return for significant reductions in their purchases of oil from the Islamic Republic.
South Korea's next review on the waiver is due in May.
Condensate imports were not included when South Korean and U.S. officials discussed cuts in Iranian crude imports, a South Korean official said.
But an executive order issued by President Barack Obama last year gives the president the option to sanction those who buy Iranian condensate.
Samsung Total has declined to comment on the deal. But a spokesman for Total in Paris said the group has complied with all measures applicable to the group.
(Reporting by Arshad Mohammed; Writing by Timothy Gardner; Editing by Warren Strobel and Xavier Briand)