South Korea's imports of Iranian crude oil plunged to the lowest in 14 months in December last year amid escalating confrontation between Tehran and Washington over the former's nuclear program, customs data showed Wednesday.

According to the data by the Korea Customs Service, the country's imports of crude oil from Iran amounted to 639,281 tons in December, sharply down from 1,196,073 tons tallied in the previous month. The amount was down from 767,051 tons in the same month a year earlier.

The December figure was the lowest since October 2010, when the country bought 528,325 tons of oil from Iran.

The plunging oil imports came amid the U.S.-led campaign to punish Iran over its nuclear program by reducing oil imports from the oil-rich country. South Korea has been under pressure to join the campaign.

Robert Einhorn, the U.S. State Department's special adviser for nonproliferation and arms control, said Tuesday during his three-day trip to Seoul that "we are urging (our partners) to reduce their purchases of crude oil from Iran and their financial dealing with the central bank of Iran."

The U.S. tough stance over Iran placed South Korea in an embarrassing situation as the Asia's No. 4 economy heavily depends on Iranian crude oil, which accounted for 9.8 percent of its combined oil needs last year.

Seoul has beefed up its sanctions over Tehran at the request of Washington. In mid-December last year, the South Korea's finance ministry blacklisted 105 Iranian firms and personnel, requiring them to receive approval from the Bank of Korea (BOK) before having any foreign currency transactions.