U.S. stocks extended declines on Thursday, hurt by a warning from General Motors about possible bankruptcy and persistent uncertainty about the fate of major banks.

Financials led the S&P down, with the financial index <.GSPF> sliding more than 8 percent, while shares of embattled bank Citigroup tumbled below $1, racking up a year-to-date drop of 85 percent.

The Dow Jones industrial average <.DJI> fell 208.92 points, or 3.04 percent, to 6,666.92. The Standard & Poor's 500 Index <.SPX> lost 25.13 points, or 3.53 percent, to 687.74. The Nasdaq Composite Index <.IXIC> was down 40.61 points, or 3.00 percent, at 1,313.13.

(Reporting by Leah Schnurr; Editing by Jan Paschal)