The European Union last week imposed an oil embargo against Iran and froze the assets of its central bank. In December, the U.S. said it would bar financial institutions from the U.S. market if they do business with Iran's central bank.

India and China are ravenous energy consumers and rely heavily on imported oil. Iranian oil accounts for 9 percent of India's oil consumption and 6 percent of China's, according to the latest data from the IEA.

Iran exports 2.5 million barrels of oil per day, about 3 percent of world supplies. About 500,000 barrels go to Europe and most of the rest goes to China, India, Japan and South Korea.

China has called for negotiations over Iran's nuclear program. South Korea has been non-committal about the sanctions, and Japan is seeking an exemption, saying its Iranian oil imports have steadily declined and probably will continue to do so.

Kyodo News agency reported that senior Japanese and U.S. officials on Thursday will hold their second meeting on the sanctions this month.