The benchmark Korea Composite Stock Price Index lost 2.22 points, or 0.09 percent, to 2,386.49 in the first 15 minutes of trading.
The decline came amid heightened tensions between North Korea and the United States.
US Air Force B-1B Lancer bombers flew in international airspace off North Korea's eastern coast in the latest show of force against the isolated country.
North Korean leader Kim Jong-un has threatened to take the "highest-level" action in response to US President Donald Trump's threat to "totally destroy" the North over its nuclear and missile programs.
Large-cap stocks were mixed across the board.
Market bellwether Samsung Electronics Co. gained 0.45 percent, and No. 2 chipmaker SK hynix Inc. was up 2.29 percent.
Meanwhile, top steelmaker POSCO was down 1.47 percent, and Naver, the operator of the country's top Internet portal, lost 1.05 percent.
The local currency was trading at 1,132.15 won against the US dollar, up 4.35 won from the previous session's close. (Yonhap)