Vitol, the world's largest oil-trading company, admitted it had worked in some aspects of the Iranian energy sector during the summer but stressed it was in compliance with international sanctions.
U.S. and European governments placed sanctions on the Iranian energy sector to try to force Tehran to be more transparent about its nuclear program. Iran says the program is for peaceful purposes.
Vitol said a subsidiary in Bahrain had purchased fuel oil from a non-Iranian counterpart in July, though the fuel oil delivered was of Iranian origin.
"Vitol Group companies no longer purchase any product of Iranian origin," the company said in a statement. "Trade with Iran never constituted a material part of Vitol's international business."
Vitol said it was in compliance with international laws and regulations regarding trade with Iran. It maintained it has ceased all sales of products to and purchases from Iran.