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Oil greased the way for U.S. stock markets to put in their worst performance in over a month on Wednesday. The Dow industrials shed 1.5 percent, or nearly 270 points, for the index's worst session since Oct. 9. The broader S&P 500 also lost more than 1.5 percent, as did the tech-heavy Nasdaq.

They were all dragged lower by plunging oil prices as concerns are growing that supply is far outstripping demand. The Organization of Petroleum Exporting Countries on Wednesday reduced its estimate for 2015 by roughly 300,000 barrels a day, with the cartel saying the effect of the 40 percent drop in prices on supply and demand is uncertain. The U.S. Energy Department fanned the flames by reporting a surprise increase in domestic oil inventories. Benchmark crude dropped 4.5 percent to close at $60.94 a barrel, the lowest since July 2009.

"OPEC saying 2015 is lower now gave oil a big chill. People are asking why is demand down, well economic activity is probably down too, so everybody is a little more cautious," said Kim Forrest, senior equity analyst at Fort Pitt Capital.


-- NBC News staff, CNBC and Reuters