The BOK said improving oil prices and increased shipments of computer chips and displays buoyed the combined sales of local companies in the January-March period.

Prices of Dubai crude -- South Korea's benchmark -- came to $53 per barrel in the first quarter, up from $31 from a year earlier.

Sales in large conglomerates climbed 8.1 percent in the first quarter from a 2.9 percent contraction in the year-ago period.

Small and medium-sized enterprises also posted a 6.7 percent increase in sales, compared with a 2.1 percent expansion a year earlier, the data said.

Korean companies' operating profit margin came to 7 percent in the first quarter, up from 5.8 percent a year earlier. It means that companies earned 70 won when they sold a product worth 1,000 won ($0.84).

The quarterly figure marked the highest since the third quarter of 2010, when it stood at 7.2 percent, the bank said.

The companies' financial health slightly improved as their assets grew 0.8 percent in the first quarter from a quarter earlier, while their debt to equity ratio fell to 90.3 percent from 91.1 percent during the same period, it said.

The BOK said its survey was conducted on 3,062 companies between April 24 and June 9. (Yonhap)