South Korean banks saw their first-half earnings jump 59 percent from a year earlier due largely to a one-off gain by state policy lender Korea Development Bank (KDB), data showed Wednesday.

The combined net profit stood at 10.8 trillion won ($9.35 billion) in the January-June period, compared with a profit of 6.8 trillion won the previous year, according to the data from the Financial Supervisory Service (FSS).

The gains came as KDB reported a jump in valuation of its stock holdings and logged higher dividend income.

In the first half, KDB reported an unusually high net profit of 2.2 trillion won after the bank converted into stocks its bond holdings of HMM, the country's largest shipper.

Excluding KDB's profits, 18 local banks' earnings amounted to 8.6 trillion won in the first half, up 2.1 trillion won from the previous year.

Local banks' interest income grew 1.7 trillion won on-year to 22.1 trillion won. Their non-interest income came to 5 trillion won, up 1.3 trillion won from a year earlier.

The average net interest margin (NIM) of the banks, a key barometer of profitability, stood at 1.44 percent in the first half, marking the second straight quarter of gains.

Local banks' NIM hit a record low of 1.38 percent in the fourth quarter of last year, affected by a long streak of low interest rates. (Yonhap)