By Caroline Valetkevitch
NEW YORK (Reuters) - U.S. stocks ended lower in a volatile session on Monday as oil prices extended their selloff, adding to worries about weak global demand.
The losses follow the S&P 500's worst weekly performance since May 2012. The index is now down 3.4 percent since Dec. 8 but is still up 7.6 percent for the year so far.
The S&P 500 energy index (.SPNY) bounced between positive and negative territory but ended down 0.7 percent after U.S. crude (CLc1) settled off 3.3 percent at $55.91, hitting fresh 5-1/2 year lows. Both U.S. crude and Brent have fallen roughly 50 percent from highs in June.
"The question of lower global demand ... and how that translates into global economic growth, is a worry for investors," said Bucky Hellwig, senior vice president at BB&T Wealth Management in Birmingham, Alabama.
Utility and financial shares were also among the day's biggest losers, with shares of JPMorgan Chase (JPM.N) down 1.5 percent at $59.16.
The Dow Jones industrial average (.DJI) fell 99.99 points, or 0.58 percent, to 17,180.84, the S&P 500 (.SPX) lost 12.7 points, or 0.63 percent, to 1,989.63 and the Nasdaq Composite (.IXIC) dropped 48.44 points, or 1.04 percent, to 4,605.16.
The S&P 500 ended below its 50-day moving average in a sign of possible further selling.
Shares of pet supply retailer PetSmart (PETM.O) rose 4.2 percent to $80.97 after it agreed to be bought by a private equity consortium led by BC Partners Ltd for $8.7 billion, in the largest leveraged buyout of the year.
Among the day's economic numbers, U.S. manufacturing output recorded its largest increase in nine months in November as production expanded across the board, pointing to underlying strength in the economy. However, the New York Federal Reserve's gauge of manufacturing turned negative in December for the first time in almost two years.
About 8.4 billion shares changed hands on U.S. exchanges, above the 7.4 billion average for the last five sessions, according to BATS Global Markets.
NYSE declining issues outnumbered advancers 2,354 to 749, for a 3.14-to-1 ratio; on the Nasdaq, 1,962 issues fell and 790 advanced for a 2.48-to-1 ratio.
The S&P 500 posted 10 new 52-week highs and 34 new lows; the Nasdaq Composite recorded 40 new highs and 179 new lows.
(Additional reporting by Rodrigo Campos; Editing by Bernadette Baum, Nick Zieminski and Meredith Mazzilli)