The KOSPI index and the won-dollar exchange rate are displayed in the dealing room of Hana Bank in Seoul, Monday. The benchmark KOSPI closed at 6,820.02, up 0.46 percent from the previous session, while the won-dollar exchange rate closed at 1,368.6 won per dollar, down 3.9 won. YonhapThe benchmark KOSPI rebounded from a steep opening drop to close higher, Monday, as share buybacks by Samsung Electronics and SK hynix lifted the index amid concerns over a potential U.S. interest rate hike in September.The won continued to strengthen, closing in the 1,360-won range against the dollar for the first time in 13 months.The KOSPI closed at 6,820.02, up 0.46 percent from the previous session. The index opened 2.58 percent lower but gained momentum as large-cap stocks rebounded broadly in afternoon trading.Foreign investors sold a net 640 billion won ($467 million) worth of shares, while institutions offloaded a net 793 billion won. Retail investors were also net sellers, unloading 144 billion won.Corporate investors net purchased 1.57 trillion won, absorbing much of the selling pressure. They extended their net-buying streak on the KOSPI to nine sessions.The buying has been supported by share repurchases from major companies, with Samsung Electronics starting a buyback on Aug. 24 as part of an employee compensation program and SK hynix purchasing about 700 billion won to 1 trillion won worth of its own shares every day since Aug. 20.SK hynix climbed 1.27 percent to 1,674,000 won, while Samsung Electronics gained 1.17 percent to reach 260,000 won.Concerns over a potential U.S. rate increase weighed on sentiment. Federal Reserve Chair Kevin Warsh struck a cautious tone on inflation at the Jackson Hole Economic Symposium in Wyoming, fueling concerns that the U.S. central bank could raise its benchmark interest rate in September.The market also faced pressure from concerns that a surge in first-half revenue at China's CXMT could signal a growing challenge to established memory-chip makers Samsung Electronics and SK hynix.Renewed military tensions between the U.S. and Iran over the weekend added to risk aversion, pushing global oil prices up more than 1 percent.The Kosdaq fell 0.49 percent to 834.29.The won-dollar exchange rate closed at 1,368.6 won per dollar, down 3.9 won from the previous session. The rate fell to its lowest level since July 24 last year, when it stood at 1,367.2 won per dollar.