On Sunday, it stopped exports to Great Britain and France. There are reports that the head of the National Iranian Oil Company is considering an expanded embargo against countries considered "hostile" to Iran.
The news sent oil prices to a nine-month high, reaching $105 per barrel in Asia on Monday.
Iran says Spain, the Netherlands, Greece, Germany, Italy and Portugal are among its European customers.
Some of these countries may no longer be importing Iranian oil. France has played down the impact of Tehran's action, saying French oil companies already have stopped purchases of Iranian crude.
Meantime, a team of U.N. nuclear inspectors has arrived in Iran to discuss that country's nuclear program. They left from Austria Monday morning.
Iran said its nuclear program is for peaceful energy purposes, but western leaders suspect the real goal is nuclear weapons.
The Associated Press contributed to this report.