The benchmark Korea Composite Stock Price Index lost 16.71 points, or 0.71 percent, to 2,351.68 as of 11:20 a.m.
The decline followed overnight losses on Wall Street. The Dow Jones industrial average fell 0.17 percent, with the tech-heavy Nasdaq composite index declining 0.28 percent.
Ryu Yong-seok, a market analyst at KB Securities Co., said tensions between North Korea and the United States are a negative factor in the local stock markets that could drag down the index.
On Thursday, North Korea's military said it is considering a plan to fire four intermediate-range ballistic missiles around the US Pacific territory of Guam to "interdict the enemy forces on major military bases on Guam and to signal a crucial warning to the US."
The North's latest threat came as US President Donald Trump warned Tuesday that North Korea will be met with "fire and fury" should it stick to ambitions to hit the US with ballistic missiles.
Most large-cap stocks were mixed across the board.
Auto parts maker Hyundai Mobis Co. was up 1.78 percent, and Naver, the operator of the country's top Internet portal, gained 0.13 percent.
Meanwhile, market bellwether Samsung Electronics Co. backtracked 0.61 percent, and top automaker Hyundai Motor was down 0.34 percent.
The local currency was trading at 1,138.95 won against the US dollar, down 3.75 won from the previous session's close. (Yonhap)