Seoul shares ended lower Thursday on heavy sell-off by foreign investors and institutions after the downgrading of the US credit rating. The South Korean won declined against the US dollar.

After choppy trading, the benchmark Korea Composite Stock Price Index retreated 0.42 percent, or 11.08 points, to 2,605.39. Trading volume was at 773.9 million shares worth 14.6 trillion won ($11.2 billion), with decliners outnumbering gainers 523 to 345.

All three US major stock indexes finished lower Wednesday (US time) as investors took profits on monthslong gains after Fitch Ratings downgraded the US credit grade to AA+ from AAA for the first time since 2011, citing "repeated debt limit standoffs and last-minute resolutions."

The July US jobs report showed private payrolls increased more than expected, indicating continued labor market resilience.

"The KOSPI turned lower on downward pressure from foreigners' spot and futures sell-off as US stock markets saw declines in tech shares after the credit downgrading," Kim Seok-hwan, an analyst at Mirae Asset Securities Co., said.

Foreign investors and institutions sold off shares worth 154.5 billion won and 670.5 billion won, respectively, while retail investors bought a total of 812.2 billion won worth of shares.

In Seoul, big-cap stocks traded mixed.

Market behemoth Samsung Electronics retreated 1.57 percent, but No. 2 chipmaker SK hynix gained 0.42 percent.

Leading battery maker LG Energy Solution dipped 0.18 percent and its smaller rival Samsung SDI went down 1.69 percent, but Posco Future M added 0.51 percent.

Steel giant Posco Holdings went up 1.2 percent.

Auto shares were mixed as well, with Hyundai Motor dropping 0.42 percent and its affiliate Kia gaining 0.88 percent. Hyundai Mobis, Hyundai Motor's auto parts affiliate, went up 1.1 percent.

Bio stocks were bullish.

Samsung Biologics jumped 2.28 percent, and Celltrion went up 2.16 percent.

Shares related to superconductors showed a big increase after researchers from the Seoul-based Quantum Energy Research Centre Inc. claimed they have developed a room-temperature superconductor, one of the most sought-after materials by science and technology experts.

Daechang, Seowon and Dcksung all shot up almost 30 percent.

The Korean won ended at 1,299.10 won against the greenback, down 0.6 won from the previous session's close. (Yonhap)