US stocks fell sharply Thursday following reports that a Malaysia Airlines plane crashed over Ukraine after being shot down.At 1551 GMT, the Dow Jones Industrial Average dropped 50.45 (0.29 percent) to 17,087.75. The broad-based S&P 500 fell 9.26 (0.47 percent) to 1,972.31, while the tech-rich Nasdaq Composite Index declined 27.97 (0.63 percent) to 4,398.00.