Seoul: Seoul shares soared nearly 6 percent Thursday as investors bet on stronger shareholder returns from chip giants Samsung Electronics and SK hynix, and eased U.S. bond market concerns. The Korean won fell against the U.S. dollar.

According to Yonhap News Agency, the benchmark Korea Composite Stock Price Index (KOSPI) rose 381.41 points, or 5.89 percent, to end at 6,852.58, after rising as high as 6,904.55. The index had previously plunged nearly 6 percent in the earlier session due to a tech sell-off but managed to recover the losses following SK hynix's announcement of a 40 trillion-won (US$28.7 billion) share buyback plan. There is also growing speculation regarding Samsung Electronics' shareholder return policy announcement later this month.

Program trading for KOSPI-listed shares was suspended for five minutes around 10 a.m. as the Korea Exchange (KRX) activated a buy-side sidecar. This measure was triggered when the KOSPI 200 Futures Index surged 5 percent or more for at least one minute.

Overnight, Wall Street's major indexes closed higher as U.S. Treasury yields fell after the Treasury Department announced plans to significantly increase buybacks of longer-term government debts. The yield on the 30-year Treasury bond, which had reached its highest level since 2007, decreased on Wednesday, along with the benchmark 10-year Treasury yield.

Trade volume was moderate, with 301.74 million shares worth 28.07 trillion won (US$20.15 billion) changing hands, as advancers outnumbered decliners 518 to 338. Foreign investors purchased a net 1.71 trillion won, while institutions and individuals sold a net 487.37 billion won and 2.28 trillion won, respectively.

"Investors returned to buying amid eased concerns over rising U.S. Treasury yields, while SK hynix's shareholder return plan further boosted market sentiment," said Kim Ki-baek, an analyst at Shinhan Securities.

Market bellwether Samsung Electronics surged 9.49 percent to 271,000 won, and its chipmaking rival SK hynix jumped 12.73 percent to 1.69 million won. SK Square, the parent company of SK hynix, advanced 11.85 percent to 1.12 million won, while Samsung Electro-Mechanics, an electronics component affiliate of Samsung Electronics, gained 0.65 percent to 1.39 million won.

The Korean won was quoted at 1,392.6 won per U.S. dollar as of 3:30 p.m., down 5.1 won from the previous stock trading session's close.