US stocks extended a brutal sell-off Thursday, plunging more than four percent in the final hours of trade, fueled by the Federal Reserve's stark warning about the health of the US economy. The Dow Jones Industrial Average was down 478.47 points (4.30 percent) to 10,646.37 at 1800 GMT. The broader S&P 500 was down 48.03 points (4.12 percent) to 1,118.73, while the tech-heavy Nasdaq Composite slumped 108.25 points (4.26 percent) to 2,429.94.