(Adds detail, background)
* S.Korea's March Iranian crude imports at 1.37 mln T
* Jan-March oil imports from Iran fall 39.4 pct to 3.45 mln T y/y
* March total crude imports at 10.91 mln T vs 12.68 mln T y/y
SEOUL, April 15 (Reuters) - South Korea's March crude oil imports from Iran dropped 39.3 percent from a year earlier on slowing demand for Iranian oil, the country's customs data showed on Sunday.
In March, the world's fifth-biggest crude oil importer, imported 1.37 million tonnes of Iranian crude, or 324,612 barrels per day (bpd), from 2.26 million tonnes from the previous year when the import volume hit a record high, according the customs data.
South Korea, one of Asia's major Iranian oil customers, mainly buys condensate, or an utlra-light oil, but its data does not provide a breakdown of imports by types.
Seoul has reduced its purchases of Tehran's oil in recent months compared to last year's levels, despite the Middle Eastern country's efforts to retain its Asian buyers by cutting official selling prices.
That came as the National Iranian Oil Company had cut supplies to South Korea by 3 million barrels each month from the start of this month due to lower production and a start of a new splitter, according to three sources.
South Korean condensate buyers were also seeking to diversify supplies should the United States impose new sanctions against Iran.
For the first quarter of 2018, South Korea's imports from Iran fell 39.4 percent to 3.45 million tonnes, or 280,736 bpd, versus 5.69 million tonnes over the same period last year, according to the data.
Meanwhile, crude imports from Qatar in January-March of this year were 2.19 million tonnes, or 178,652 bpd, up 2.4 percent from 2.14 million tonnes during the same period last year. South Korea typically switches to Qatari condensate to replace Iranian one.
Overall, South Korea's March crude oil imports were 10.91 million tonnes, or 2.58 million bpd, down 14 percent from 12.68 million tonnes from last year, according to the data.
In the first three months of this year, South Korea imported 36.70 million tonnes of crude, or nearly 3 million bpd, down 0.4 percent from 36.88 million tonnes from the previous year.
The country's final data for March crude imports data will be released by state-run Korea National Oil Corp (KNOC) later this month.
(Reporting by Jane Chung; Editing by Sam Holmes)