The South Korean won weakened on Wednesday as importers bought dollars and foreign investors sold local shares as Seoul's main index fell.
Investors also covered dollar short positions that had been built earlier as exporters sold the US currency for the won and on concerns that China may let the yuan appreciate further.
The won ended 0.6 percent lower, the biggest daily decline by percentage since dropping 0.63 percent on February 22. The won finished locally at 960.4 per dollar versus Tuesday's domestic close of 954.7. It hit a session high of 952.6, dealers said.
"Investors had preferred dollar short positions in the morning but abruptly reversed their stances as the won broke through 955, hit by offshore purchases," a local bank dealer said. "So the won extended its losses in the afternoon."
Offshore players bought the US currency as foreign investors sold a net 178.6 billion ($186 million) on Seoul's main Kospi index, which closed 0.18 percent lower as oil prices rose, dealers said.
Investors were also wary about possible intervention by foreign exchange authorities given the won's recent appreciation against the Japanese yen.
The yen, which the won usually follows closely as South Korean firms compete with Japanese peers in key export markets, was quoted at 118.06/07 per dollar, up from 118.43/45 in late Asian trade on Tuesday.
The dollar edged down, slipping further from a one-month high against the yen as dealers turned cautious ahead of US trade data due later on Wednesday and on revived geopolitical concerns after Iran announced that it possessed nuclear technology.
The United States was likely to report that its trade deficit narrowed to $67.50 billion in February from the previous month's record shortfall of $68.51 billion, supported by softer oil prices and weaker US consumer purchases that helped reduce imports, according to a Reuters survey.