South Korea's terms of trade worsened in the second quarter from three months earlier due to rising costs for crude oil imports, the central bank said Monday.

According to the Bank of Korea (BOK), South Korea's net terms-of-trade index for goods fell 3.6 percent to 71.3 in the April-June period, compared with 74.0 in the first quarter.

"Per-unit import costs grew at a faster pace than per-unit export prices due to a hike in international crude oil prices," said a statement by the BOK.

The index for per-unit import costs rose 5.2 percent quarter-on-quarter to 130.6 in the three-month period as prices of raw materials, machinery and electronic equipment increased, the BOK said.

The index for per-unit export prices increased 1.3 percent to 93.1 on higher petroleum and light-industry product prices, the statement added.

The average price of Dubai crude oil, South Korea's benchmark, climbed to 65.85 U.S. dollars a barrel in June from 63.98 U.S. dollars in April.

The index is calculated by dividing the export price index by the import price index. The base year is 2000 with a benchmark index of 100.