South Korean banks' combined earnings tumbled 25.3 percent in the first quarter of the year from a year earlier due to shrunken interest income and profitability, the financial watchdog said Thursday. The combined net income of 18 local banks registered at 1.3 trillion won (US$1.26 billion) in the first three months of the year, compared with 1.7 trillion won the previous year, according to the Financial Supervisory Service (FSS).