Standard & Poor's late Wednesday revised its outlook on the unsecured debt issuances of Fannie Mae, Freddie Mac and the Federal Home Loan Bank (FHLB) System to "negative" from "stable" while maintaining their 'AAA' credit rating.

The action follows a similar revision the credit rating agency took earlier this week with respect to U.S. government debt.

S&P said the ratings of the government sponsored enterprises are "constrained by the long-term sovereign rating" on the U.S.

"We will not raise the outlook and ratings of these [GSEs] as long as the ratings and outlook of the U.S. remain unchanged," S&P said.

Fannie and Freddie have been operating under a federal conservatorship since the fall of 2008. Both have positive net worth positions but only because the U.S. Treasury Department has bolstered their net worth positions.

To date, Treasury has provided roughly $145 billion of government support to Fannie and Freddie. Both posted an operating profit in the fourth quarter.

The FHLBs, on the other hand, have required no government assistance and are not in conservatorship.

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