Korea’s imports of crude oil from Iran plunged in December amid growing demand from the U.S. to toughen sanctions on Tehran over its suspected nuclear weapons program, customs data showed Wednesday.

Korea bought about 639,000 tons of Iranian oil in December, down 46.5 percent from a month earlier, according to the data by the Korea Customs Service. The amount is down 16.7 percent from the same month a year earlier.

The December figure was the lowest level since October 2010, when Seoul imported 528,000 tons of crude oil from Iran.

Exact causes were not provided but experts attributed the plunge to actions by major refinery firms to cut oil purchases from Iran ahead of an expected government move to toughen sanctions on the country.

South Korea is under pressure from the U.S., which asks its allies to join in punishing Iran for seeking to develop nuclear technologies. Tehran claims its nuclear aspirations are for peaceful purposes.

Late last month, U.S. President Barack Obama signed into law a bill that imposes tough sanctions against financial institutions dealing with the central bank of Iran.

On Tuesday, Robert Einhorn, the U.S. State Department’s special adviser for nonproliferation and arms control, held talks with South Korean policymakers, calling for Seoul’s cooperation.

Washington’s toughened stance on Iran has placed South Korea in a difficult position as Seoul depends on Iranian crude oil for much of its energy needs.

Last year, South Korea imported 9.8 percent of its crude oil needs from Iran, which amounted to 12.42 million tons. The ratio jumped to as high as 12 percent in November before plunging to 5.9 percent in December. (Yonhap News)

Some industry watchers said the decline might have been caused by a one-off factor related to the amount of inventory in the refinery sector. The government denies any pressure on refineries to cut their imports of Iranian crude oil.

The decline, however, comes as South Korea has been beefing up its sanctions on Tehran at the request of the U.S. In mid-December, Seoul announced fresh sanctions, limiting financial deals with 99 Iranian groups and six individuals from the Middle Eastern country. 

(Yonhap News)