An Iranian tanker with a Tanzanian flag was turned away when it sought to refuel at an island offshore Athens after its real nationality was discovered, a Greek fuel supplier said Wednesday, illustrating how Western sanctions against Iran continue to impede the Islamic Republic's efforts to keep trade flowing.

The incident also highlights how European companies are at risk of unwittingly breaching sanctions, which bar the refuelling of Iranian ships, because of constant changes of flags, names and nominal owners of Iran's tankers amid international pressure.

Yannis Spyridakis, operations manager at Greek shipping fuel supplier Sekavin SA, said his company had been asked to refuel a crude-oil tanker on Syros Island, which is located 78 nautical miles (144 kilometers) south-east of Athens. The vessel identified itself only as having a Tanzanian flag, but Sekavin found out it was owned by the National Iranian Tanker Co. after conducting some checks, Mr. Spyridakis told Dow Jones Newswires.

Refuelling the Iranian tanker would have been illegal under EU sanctions, "that's why we didn't refuel it," he said.

Vessels "declare their flag only, they are not obliged to declare their owner" when they dock at a port, Mr. Spyridakis said. In the case of NITC, "most of their fleet changed flags in the past months," he said.

The vessel involved, the Baikal, has switched its registration from one country to another this year to avoid the EU ban on Iranian oil shipping, moving from Malta to the Pacific island nation of Tuvalu. Earlier this year, it was barred from the Tuvalu Ship Registry following U.S. pressure. It now flies a Tanzanian flag, though the East African nation has previously denied registering it.

NITC officials weren't available for comment.

European Union bans on shipping and insuring Iranian oil since July have forced Tehran to increasingly rely on NITC for its oil shipments, after most international shippers stopped working with the country. As Brussels seeks to intensify pressure on the Islamic Republic over its disputed nuclear program, the EU last month barred companies on its territory from dealing with NITC, which is Iran's main oil tanker operator.