* Indexes reverse course after higher open
* Crude resumes slide
* Pandora soars after comments on new music royalties
* FedEx rises after Q3 profit beats view
* Indexes down: Dow 0.82 pct, S&P 0.99 pct, Nasdaq 0.77 pct (Updates to early afternoon)
By Abhiram Nandakumar
Dec 17 (Reuters) - Wall Street looked set to snap a three-day rally on Thursday, dragged down by energy and materials stocks, a day after the Federal Reserve raised interest rates for the first time in nine years.
Crude oil prices resumed their slide after gaining earlier in the day on persistent oversupply worries and as the dollar hit a two-week high. Exxon was down half a percent, while Chevron slid 1.4 percent.
Fed Chair Janet Yellen's assurance that further tightening would be gradual boosted global stock indexes.
However, investors remain concerned about weak global economic conditions as the slide in commodity markets continues unabated and demand slows.
"Investors continue to struggle with the notion of the strength in the U.S. economy, in the context of this performance within global conditions that are mixed at best," said Mark Luschini, chief investment strategist at Janney Montgomery Scott in Philadelphia.
At 12:31 p.m. ET (1731 GMT), the Dow Jones industrial average was down 145.22 points, or 0.82 percent, at 17,603.87, the S&P 500 was down 20.62 points, or 0.99 percent, at 2,052.45 and the Nasdaq Composite index was down 39.29 points, or 0.77 percent, at 5,031.85.
All 10 major S&P sectors were lower, with the energy sector's 2 percent fall leading the decliners. Conocophilips was down 4.4 percent and was the biggest drag on the sector.
Newmont Mining was down 8 percent at $17.58 and was the biggest influence on the materials index's 1.25 percent decline.
Apple was down 1.2 percent and was the biggest drag on the S&P and the Nasdaq, while Goldman Sachs' 1.3 percent decline weighed on the Dow.
General Mills was down 3.2 percent at $56.31 after its quarterly sales missed estimates.
Oracle was down 5.9 percent at $36.63 after its third-quarter profit forecast failed to impress investors.
Pandora was up 13.6 percent at $15.31 after the media-streaming company said new music royalty rates were "balanced".
FedEx was up 3.2 percent at $153.57 after it reported a better-than-expected quarterly profit.
Declining issues outnumbered advancing ones on the NYSE by 1,892 to 1,086. On the Nasdaq, 1,716 issues fell and 1,013 advanced.
The S&P 500 index showed 10 new 52-week highs and 17 new lows, while the Nasdaq recorded 41 new highs and 70 new lows. (Reporting by Abhiram Nandakumar in Bengaluru; Editing by Anil D'Silva)