The benchmark Korea Composite Stock Price Index fell 2.83 points, or 0.12 percent, to 2,369.74 in the first 25 minutes of trading.
The main index continued to fall for the eighth consecutive session on foreign and institutional selling. Foreigners dumped more than 1 trillion won ($876 million) worth of stocks in the past seven sessions due mainly to the North Korea risks.
US President Donald Trump said Tuesday he is "totally prepared" to use the military options against North Korea. His latest warning came in response to Pyongyang's threat a day earlier to shoot down approaching American bombers, even if they are in international air space.
Large-cap stocks were mixed across the board.
Top carmaker Hyundai Motor Co. fell 1.69 percent, leading steelmaker POSCO shed 0.96 percent, and state-run utility Korea Electric Power Corp. was down 0.52 percent.
Among gainers, market bellwether Samsung Electronics Co. moved up 1.28 percent, and No. 2 chipmaker SK hynix Inc. jumped 3.16 percent.
The local currency was trading at 1,144.05 won against the US dollar, down 3.35 won from the previous session's close. (Yonhap)