India, Turkey, South Korea, Sri Lanka, Taiwan, Malaysia and South Africa were given exemptions last year from U.S. sanctions targeting the Iranian oil sector because of reductions in crude oil imports from the Islamic republic.
South Korea stopped importing oil from Iran in August following a ban imposed by the European Union. Imports resumed in October, however, when Iran offered up its own vessels to transport crude oil.
The latest customs data from the South Korean government indicate crude oil shipments from Iran declined by about 17 percent in March, Bloomberg News reports.
U.S. Secretary of State John Kerry last month said Japan and 10 EU countries qualified for an exception to Iranian sanctions for moving away from Tehran's oil sector.
Japan, however, said it purchased about 321,000 barrels of Iranian crude oil per day in February. That's about 16 percent higher than the same time last year and the highest level since March 2012, Bloomberg added.
The U.S. Energy Department last month reported "unprecedented drops" in Iranian crude oil exports because of U.S. and European sanctions imposed on the energy sector.