South Korea's terms of trade have deteriorated as rapidly rising global oil prices are pushing up import costs. 

The amount of imports that can be purchased by a single export unit has plunged to the lowest level in three and a half years.

According to Bank of Korea data on trade indexes and trade terms for the month of May, the terms of trade index for products came to 93-point-23 last month, down five-point-three percent from a year earlier.

It's the lowest reading since the end of 2014 and the steepest on-year drop since April 2012 when the index lost seven-point-five percent.

The terms of trade index for products measures the amount of imports a country can purchase for each unit of its exports.

The falling index is in big part due to rising global oil prices which soared 46-point-seven percent last month from a year ago.