South Korea's KOSPI is plunging more than 5 percent on weakness in heavyweight semiconductor shares. Profit-taking after a sharp jump the previous day and net foreign selling coincided, triggering the year's 14th sell sidecar on the Korea Stock Exchange.

As of 11:18 a.m. on Thursday, the KOSPI was down 483.76 points, or 5.42 percent, at 8,446.54. The index opened down 117.12 points, or 1.31 percent, at 8,813.18, before extending losses.

On the main board, retail investors were net buyers of 283.27 billion won. Foreigners and institutions were net sellers of 243.46 billion won and 43.77 billion won, respectively, dragging the index lower.

With the KOSPI sliding sharply, the main board triggered a sell sidecar that temporarily halts the effectiveness of program sell orders. It was the 14th time this year that a sell sidecar has been triggered on the main board.

Among top stocks by market capitalisation, semiconductor shares posted the biggest declines. Samsung Electronics was trading at 336,000 won, down 22,500 won, or 6.28 percent, from the previous session. SK Hynix was down 205,000 won, or 7.03 percent, at 2,712,000 won.

Other large caps were also lower, including SK Square, down 9.43 percent, Hyundai Motor, down 4.17 percent, Samsung Life Insurance, down 3.36 percent, Samsung C&T, down 2.70 percent, LG Energy Solution, down 4.26 percent, and Samsung Biologics, down 3.03 percent. Samsung Electro-Mechanics was up 0.75 percent.

The Kosdaq was also weak. At the same time, the Kosdaq was down 21.97 points, or 2.47 percent, at 865.84.

The won-dollar exchange rate was rising. The morning won-dollar rate was 1,549.55 won, up 4.25 won, or 0.27 percent, from the previous session.

The securities industry sees concerns over cost burdens for demand-side companies from a sharp jump in memory prices as having spurred profit-taking in semiconductor shares.

After Micron's earnings release, expectations for an improvement in the memory industry remain intact. But concerns have also surfaced that higher semiconductor prices could squeeze margins at major demand-side companies such as Apple and Microsoft.

The market cites whether semiconductor shares can see a recovery in supply and demand and the intensity of foreign selling as factors that will determine the index's direction going forward. Weakness in heavyweight stocks such as Samsung Electronics and SK Hynix is pressuring the broader index.