* Won hits lowest since Feb. 5     * Won's losses seen capped at 1,080 level      By Christine Kim     SEOUL, March 21 (Reuters) - The South Korean won  
  KRW=
   stumbled for a fourth day and fell to its lowest in more than six weeks early in Friday's session, pressured by the U.S. dollar for a second day after Federal Reserve chair Janet Yellen hinted at a sooner-than-expected tightening of rates.     The local currency  
  KRW=KFTC
   was quoted at 1,079.0 against the dollar as of 0230 GMT, down 0.3 percent from Thursday's domestic close at 1,076.2.     The won fell as far as 1,080.5 shortly after markets opened, its lowest since Feb. 5, but losses were capped quickly by dollar sales from local exporters, market participants said.      "Today's trade flow depends on how the tug-of-war will turn out between local exporters and offshore players," said a bank trader in Seoul.     The greenback broadly firmed for a second day on Friday after Yellen suggested the first increase in interest rates in the U.S. could come in the first half of 2015 - considerably  earlier than many had expected.     "There are few catalysts that could weaken the won further, aside from the U.S. dollar's broad strength or a softening yuan  
  CNY=
  ," Hong Seok-chan, a foreign-exchange analyst at Daishin Securities, said in a note to clients.     Hong also said the won's losses would likely be capped at the 1,080 level for the time being.     The benchmark Korea Composite Stock Price Index  
  .KS11
   was up 0.6 percent.     June futures on three-year treasury bonds  
  KTBc1
   edged down 0.03 points to trade at 105.71.                             0230 GMT    Prev close  Dollar/won            1,079.0       1,076.2  Yen/won           10.5393/463       10.5085  *KTB futures           105.71        105.74  KOSPI                1,931.31      1,919.52  * Front-month futures on three-year treasury bonds   (Additional reporting by Yena Park; Editing by Eric Meijer)  ((christine.kim@thomsonreuters.com)(822 3704 5665)(Reuters Messaging: christine.kim.thomsonreuters.com@reuters.net))  Keywords: MARKETS KOREA FOREX BONDS/