Standard & Poor's Ratings Service lowered its outlook for both Bank of America Corp. and Citigroup Inc. to negative, saying it is unsure whether the U.S. government would be willing to give additional support to the banks in a way that benefits their debtholders.

"We believe markets are beginning to stabilize and the U.S. government is seeking ways to reduce the potential for moral hazard and systemic risk associated with large financial institutions," the company said.

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