South Korea, one of Iran's main Asian customers, imported 1.41 million tonnes of Iranian crude in April, or 344,933 barrels per day (bpd), customs data showed on Monday. That was down 37.5 percent from nearly 2.3 million tonnes in March, but still up 63.5 percent from 863,557 tonnes a year earlier.
Iran has sold all the crude that it stored at sea during years of sanctions over its contentious nuclear programme, shipping and oil sources said last month, ISNA reported.
Since sanctions were eased in January 2016, Iran has been ramping up oil output to try and regain its lost market share.
South Korea, the world's fifth-largest crude importer, brought in about 7.1 million tonnes, or 433,659 bpd, of crude from Iran in the first four months of the year, up 86 percent from 3.82 million tonnes a year ago.
South Korea's crude imports from Saudi Arabia, the world's top crude exporter, in April fell 15.1 percent to 3.56 million tonnes, or 859,127 bpd, from a year ago.
The Organization of the Petroleum Exporting Countries (OPEC) and other producers including Russia agreed to cut output by almost 1.8 million barrels per day in the first half of this year. Producers will meet on May 25 to discuss extending the cuts.
Overall, South Korea's crude oil imports in April fell 5.6 percent to 11.31 million tonnes, or 2.76 million bpd, from a year earlier, the data showed.
For the January-April period, the country imported 48.25 million tonnes of crude, or 2.95 million bpd, up 2 percent from 47.3 million tonnes a year ago.