SK Innovation, South Korea's No.1 oil refiner, recorded a 25 percent drop in operating profit for the third quarter from a year earlier due to a rise in oil import prices, a regulatory filing showed Friday.

Consolidated operating profit reached 648.7 billion won ($592 million) during the July-September period, down 24.7 percent from the same period of last year, according to the regulatory filing.

Sales expanded 7.5 percent on-year to 18.5 trillion won in the three months ending September 30, but net income tumbled 71.9 percent to 509.7 billion won.

The decline in earnings was attributable to rising prices in crude imported from overseas. "Following the half of oil imports from Iran, the proportion of Brent crude import increased during the quarter, raising production costs," said Kim Tae-hee, an analyst at Dongbu Securities in Seoul.

Kim, however, noted that resumption of oil imports from Iran was expected to boost operating performance in the fourth quarter, saying that the company's refining margin has stayed at a high level.