Business sentiment among South Korean manufacturers turned pessimistic for the fourth quarter, falling to the lowest in 2-1/2 years, possibly weighing on investment and business plans for next year, a private survey showed.

The Korea Chamber of Commerce and Industry (KCCI) said in a statement on Sunday that its manufacturers' business survey index dropped to 94 for the October-December quarter, breaking below the boom-or-bust threshold of 100.

The index stood at 103 for the third quarter and last tumbled below the 100 mark for the second quarter of 2009 to 66.

South Korea's trade-dependent economy leaves it vulnerable to external volatility, and global market jitters over the fragile U.S. economy and the euro zone debt crisis have shaved 4.5 percent off the won in the past week to a one-year low.

The industry group, which represents more small- and medium-sized enterprises (SMEs) than large conglomerates, polled 2,000 manufacturers across the country between August 16 and 31.

SMEs made up 91 percent of the surveyed companies.

As to the possibility of the global economy entering a double-dip (recession), 68 percent of respondents said it was highly likely, KCCI said in the statement.

(Reporting by Tae-Yi Kim and Kim Yeonhee; Editing by Chris Lewis)