The benchmark Korea Composite Stock Price Index shed 1.24 points, or 0.05 percent, to 2,414.81 as of 11:20 a.m.
The two-day Federal Open Market Committee meeting is slated to finish Wednesday (local time) probably providing clues on US monetary policy.
Most large caps on the Seoul bourse traded mixed, with market kingpin Samsung Electronics shedding 0.12 percent.
SK hynix, a major chipmaker, was up 0.51 percent.
LG Chem, South Korea's top chemicals firm, surrendered 2.02 percent, while AmorePacific, the country's No. 1 cosmetics maker, gained 0.99 percent.
Carmakers were among the gainers with industry leader Hyundai Motor climbing 1.07 percent and its smaller affiliate Kia Motors also advancing 1.38 percent.
The local currency was trading at 1,128.80 won against the US dollar, up 2.50 won from the previous session's close. (Yonhap)