* U.S. Treasury places S.Korea on new monitoring list * KOSPI set to mark a fourth losing session SEOUL, May 2 (Reuters) - The South Korean won opened lower on Monday after the U.S. Treasury put the country on a new monitoring list in a biannual report on exchange rates, but the currency pared losses by midday. The won was quoted at 1,139.8 per dollar down 0.1 percent compared to Friday's close of 1,139.3. The Treasury report said South Korea has a significant bilateral trade surplus and Korean authorities had intervened in foreign exchange markets but it did not meet standards that called for enhanced analysis. South Korea has no plan to change its current stance on smoothing plans in case of sudden one-sided changes in foreign exchange trading, a foreign exchange authority told Reuters on Monday. "Although the foreign exchange authorities say they will keep engaging in smoothing operations, they won't be able to as much as before following the U.S. report," said Jung Sung-yoon, a foreign exchange analyst at Hyundai Futures. Jung added exporters' dollar sales were likely to support the won on Monday. South Korean shares slumped as foreigners and domestic institutions sold stocks. The Korea Composite Stock Price Index (KOSPI) was down 0.8 percent at 1,979.13 points. Shares looked to to post their four straight losing session. Offshore investors were set to be sellers, offsetting 52.2 billion Korean won ($45.80 million) worth of KOSPI shares near mid-session, weighing on the index. Decliners far outnumbered advancers 591 to 209. The sub index for iron, steel and metal underperformed the broader market, with major steelmaker Posco down 5.2 percent and Hyundai Steel Co down 3.8 percent. Market heavyweight Samsung Electronics Co Ltd gained 0.2 percent, after falling 4.2 percent in the last two sessions. June futures on three-year treasury bonds lost 0.01 point to 110.33. 0243 GMT Prev close Dollar/won 1,139.8 1,139.3 Yen/won 10.7005/63 10.7563 *KTB futures 110.33 110.34 KOSPI 1,979.13 1,994.15 * Front-month futures on three-year treasury bonds (Reporting by Dahee Kim; Editing by Kim Coghill)