New Delhi, July 28 -- Whatever you do this morning, don't look at the KOSPI Index.

South Korea's blue-chip index, the world's hottest stock market over the past year, is rattling Wall Street nerves again with a sharp overnight tech selloff. It suggests mounting concerns for the market's biggest driver heading into a crucial stretch of the second-quarter earnings season.

The KOSPI fell the most since early spring, dragging the benchmark to its worst monthly performance since the Asian financial crisis of 1997, on reports of a China-based firm ramping production of a high-end chipmaking machine.

Japan's Nikkei 225 also slumped-taking it deeper into correction territory and now marked 13.8% south of its late June levels-on concerns tied t...