Ratings agency Standard & Poor's said late Thursday that it has placed its 'AAA' long-term and 'A-1+' short-term sovereign credit ratings on the United States of America on CreditWatch with negative implications.

S&P also said that it may lower the long-term rating on the U.S. by one or more notches into the 'AA' category in the next three months, if the agency conclude that Congress and the President Barack Obama's Administration have not achieved a credible solution to the rising U.S. government debt burden and are not likely to achieve one in the foreseeable future.

"Owing to the dynamics of the political debate on the debt ceiling, there is at least a one-in-two likelihood that we could lower the long-term rating on the U.S. within the next 90 days," S&P said.

S&P's warning came a day after Moody's warned of a possible downgrade of the U.S. government debt rating.

Moody's Investors Service said late Wednesday it is placing the "Aaa" government debt rating of the U.S. under review for a possible downgrade.

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