* Brent crude hits lowest level since July 2009
* Airline shares up; boosted by lower oil prices
* Indexes off: Dow 1 pct, S&P 0.9 pct, Nasdaq 0.7 pct (Updates to late morning)
By Caroline Valetkevitch
NEW YORK, Dec 10 (Reuters) - U.S. stocks extended their declines in afternoon trading on Wednesday as the energy sector resumed its recent slide along with oil prices.
The S&P energy index was down 3.3 percent, the worst performing S&P sector and on track for its fourth drop in five sessions. Brent crude touched a new five-year low of $63.56 on signs of oversupply and soft demand.
Exxon Mobil, down 3.2 percent, led declines on the S&P 500.
Falling oil prices have raised concerns about earnings for energy companies, with year-end tax selling putting additional pressure on the group. The sector, down more than 15 percent for the year so far, is the worst performing of the 10 major S&P sectors for the year.
"Oil seems to be the one theme that has more control over the direction of the market and the degree that volatility emerges than any other theme right now," said Peter Kenny, chief market strategist at Clearpool Group in New York.
Shares of Yum Brands Inc dropped 4.9 percent a day after it lowered its profit forecast for the year for the second time, hurt by slower-than-expected sales recovery in China following a food safety scare in July.
At 1:43 p.m., the Dow Jones industrial average fell 169.64 points, or 0.95 percent, to 17,631.56, the S&P 500 lost 17.52 points, or 0.85 percent, to 2,042.3 and the Nasdaq Composite dropped 32.99 points, or 0.69 percent, to 4,733.48.
Among the day's gainers were airline shares, including Southwest Airlines, up 4.4 percent and was leading gains on the S&P 500. Barclays said airlines stand to see a decline of about $10 billion in fuel costs in 2015.
Declining issues outnumbered advancers on the NYSE by 2,343 to 701, for a 3.34-to-1 ratio; on the Nasdaq, 1,904 issues fell and 743 advanced for a 2.56-to-1 ratio.
The S&P 500 was posting 32 new 52-week highs and 24 lows; the Nasdaq Composite was recording 84 new highs and 67 lows. (Additional reporting by Chuck Mikolajczak; Editing by Chizu Nomiyama and Nick Zieminski)