For the whole of 2017, South Korea's oil imports from Iran rose 29.1 percent to 18.07 million tons, or 362,884 bpd, versus 14 million tons in 2016, reported Reuters quoting customs data.
South Korea, the world's fifth-largest crude importer in December, brought in 1.08 million tons of Iranian crude, or 255,081 barrels per day (bpd), down 30.6 percent from 1.55 million tons a year earlier.
Major Asian crude buyers including South Korea have snapped up purchases of Iranian oil since sanctions over the nation's nuclear program were lifted a couple of years ago.
South Korea mainly buys condensate — an ultralight form of crude — from Iran, but its data does not provide a breakdown on categories of imports.
Despite a push by Iran to boost exports, its condensate shipments have been dented by a technical problem at the South Pars oilfield.
Iranian oil and shipping sources said earlier this month that oil output from the third-biggest producer in the Organization of the Petroleum Exporting Countries (OPEC) had stayed around 3.8 mbd.
In total, South Korea imported 13.25 million tons of crude oil in December, or 3.13 mbd, up 0.1 percent from 13.24 million tons a year ago, the data showed.
South Korea's crude oil imports from Saudi Arabia last month edged down 0.2 percent from a year ago to four million tons, or 946,615 bpd, with total 2017 crude intake from its top oil supplier falling 1.7 percent to 43.06 million tons from 2016.
In full-year 2017, South Korea's crude oil imports increased 3.3 percent to 148.68 million tons, or 2.99 million bpd, versus 130.71 million tons the previous year.
Final data for South Korea's December crude imports data will be released by state-run Korea National Oil Corp. (KNOC) later this month.