The figures show Turkey, the world's fifth-largest buyer of Iranian oil in 2011, is holding to its pledge to cut imports from the Islamic Republic after buying and stockpiling more Iranian oil from January through April than it did last year.
Before May, Turkey was the only buyer in Europe to increase purchases from Iran, while other European refiners cut back on imports of the crude ahead of an impending EU oil embargo due to take effect from July 1.
The latest round of US sanctions come into effect on June 28 to pressure Tehran into halting its nuclear program. The United States this week is due to announce a list of countries it will exempt from financial sanctions on oil trade with Iran if they make significant reductions to the crude imports.
In the first four months of 2012, Turkey imported 210,000 barrels per day of Iranian oil on average, including a huge 270,000 bpd in March, much higher than its 2011 average of 185,000 bpd.
In May Turkey's state-controlled refining company, Tupras, imported around 140,000 barrels per day (bpd), a 20 percent drop from its 2011 average, according to the latest shipping data, obtained by Reuters.
Port data showed 152,000 tons of Iranian crude was delivered to the port of Aliaga in May, while 443,000 tons of Iranian crude was delivered to its second import terminal, Tutunciflik.
Tupras is expected to import the same volume in June. From July 1, Turkey will remain effectively the sole buyer of Iranian crude in Europe.
Official trade data showed that in the first four months of this year, Iran accounted for about 58 percent of Turkey's near 6 million tons in total crude imports.
The EU and US widened sanctions at the start of the year on suspicions that Iran is trying to develop atomic bombs, while Iran insists its nuclear program is solely for civilian purposes.