Standard & Poor's cut its ratings outlooks for Bank of America [BAC] and Citigroup [C] to "negative," from "stable," saying Tuesday that it is uncertain whether the government will keep giving "extraordinary" support to the banking giants.

S&P cited recent legislation that would halt bailouts of specific companies as an example of how the government may scale back support of the financial industry, to the potential detriment of bondholders.

The ratings agency's analysts said in a conference call that credit losses at the two companies could remain high and that they were not convinced that U.S. banks had reached a turning point in the downturn.