The East Asian country is the largest importer of Iranian condensate, an ultra-light oil which has a wide range of utilization in the petrochemical industry.
According to customs data, South Korean refiners imported 1.45 million tonnes of crude oil from Iran in April, compared to 1.24 million tonnes a year earlier.
South Korea said last month it was sending a delegation to the United States to query American authorities about their decision to end waivers for imports of Iranian oil.
According to Reuters news agency, Seoul has told US officials that there are few options for getting the same quality of Iranian condensate from other suppliers.
Asian customers favor Iran's South Pars condensate for its rich yield of naphtha which is a key feedstock for petrochemicals and a gasoline blending component.
Before US sanctions, South Korea was the biggest client of Iranian condensate with 300,000 bpd on top of 100,000 bpd of crude oil.
However, the United States is reportedly pushing its fast-growing condensate on South Korea, for which Seoul has to reconfigure it refineries which are built to process Iranian grades.
Wednesday's customs data showed South Koreas crude oil imports from the United States grew five-fold to 1.45 million tonnes in April, or 353,006 bpd, from 268,511 tonnes a year earlier.
The United States is aggressively pushing forth with Trumps energy dominance agenda that seeks to advance diplomatic and policy objectives through rapidly expanding US oil and gas exports.
The policy is disrupting markets and complicating US relations with many countries, including own partners. Major oil importers, especially Asian customers of Iranian oil such as China, India, South Korea and Japan, are seeing their energy security being increasingly encroached on by the US.
Beijing has vehemently denounced the US implementation of unilateral sanctions and its so-called long-armed jurisdiction against Iran and Venezuela.
China said last month US sanctions would intensify the turmoil in the Middle East and the international energy market.
Turkey, another importer of Iranian oil, also denounced the US bid to bring Irans oil exports to zero as dangerous.
Last week, the US sent the USS Abraham Lincoln aircraft carrier and its associated strike group to the Middle East.
The deployment was followed by mysterious attacks on commercial vessels in the Persian Gulf, which sent oil prices surging by more than a dollar above $71 a barrel.
Stock markets in the Persian Gulf also suffered their worst declines in years as the US escalation increased regional uncertainty which is feeding into investment jitters.
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