Asia's fourth-largest economy brought 1.4 million tonnes or 114,115 bpd of crude from the Middle Eastern country in the first three months of this year, below 136,281 bpd in the same period of last year and last year's average of 125,000 bpd.

Iranian crude shipments in 2014 were 6.2 million tonnes, or 124,497 bpd, down 7.1 percent from the 2013 average of 134,000 bpd, according to the data and Reuters calculations in January.

Under current sanctions, big Asian buyers, including South Korea, should hold their crude imports from Tehran at end-2013 levels. Of South Korea's four refiners, only SK Energy Co Ltd and Hyundai Oilbank Co Ltd import Iranian oil, and their imports fluctuate each month.

Overall, South Korea imported 11.05 million tonnes of crude last month, or 2.61 million bpd. That was 8.7 percent higher than the 10.16 million tonnes imported in March of 2014, the customs data showed.