South Korea's listed firms were expected to post weak sales, but their operating profit will sharply improve this year thanks to lower oil prices and restructuring efforts following last year's slump, data showed Wednesday.

The combined sales of 237 firms listed on the main stock market was estimated at 437 trillion won ($398 billion) in the first quarter, down 5.38 percent from a year ago, according to the data compiled by the Korea Exchange.

Except for a 0.07 percent hike in the third quarter, their sales were expected to slip 3.1 percent and 1.4 percent in the second and fourth quarters, data showed.

Despite the downbeat outlook, major companies expected better operating incomes down the road on the back of low oil prices.

Of 164 companies that released 2015 earnings guidances, their combined operating profit was estimated to have decreased 4.1 percent on-year to 24.8 trillion won in the January-March period, but their earnings are expected to advance 5.5 percent in the April-June period.

Their earnings growths are forecast to further accelerate 23.5 percent and 22.8 percent in the third and fourth quarters, respectively, data showed.

"Sales growth remains slow due to the weaker-than-expected economic recovery, but companies are expected to enhance profits via restructuring efforts and reduced raw material costs," said Yang Hae-jung, a strategist at E-Best Securities. "Earnings will increase thanks to a higher margin ratio despite lackluster sales growth."

Some said the improved profits helped by low costs will be limited as domestic and global demand have not yet fully recovered to support sustainable growth.

"Tepid sales mean that local companies haven't been able to diversify their income sources," said Lee Han-deuk, a researcher at the LG Economic Institute. "Risks that remain as external factors can change very quickly." (Yonhap)