July 11 (Bloomberg) -- The U.S. would retain its top AAA credit ratings even if the government was forced to rescue mortgage lenders Fannie Mae and Freddie Mac, according to Moody's Investors Service Inc. and Standard & Poor's.

U.S. debt is ``well within'' the guidelines for an Aaa rating, said Steven Hess, vice president and senior credit officer at Moody's in New York. The U.S.'s AAA rating is not at risk, said Nikola Swann, S&P's primary U.S. credit analyst.

``Even under a real stress scenario, the amount of money the government would have to come up with is not that large,'' Moody's Hess said. ``The amount of money required would not be so large that it would make us worry about the U.S. credit rating.''

Treasuries fell as speculation the U.S. government won't allow the two biggest U.S. mortgage lenders to fail reduced demand for government debt as a haven from turmoil in the credit markets. Treasury Secretary Henry Paulson, seeking to reassure investors after Fannie Mae and Freddie Mac slid to their lowest levels in more than 17 years, said officials want to keep the firms in their current form.

``Going from an implicit guarantee to an explicit guarantee doesn't seem to be a big change,'' S&P's Swann said. ``Our view is that a capital injection is not likely. We think their assets are sufficient to cover their liabilities.''

In April, S&P said the possible need to provide financial support to government-sponsored enterprises that deal in mortgage debt poses a greater threat to the AAA credit rating of the U.S. than bailouts of securities firms.

`Misleading'

The maximum potential cost of assisting broker-dealers in a prolonged recession is less than 3 percent of gross domestic product, compared with as much as 10 percent to agencies such as Freddie Mac, Fannie Mae and Federal Home Loan Banks, S&P said in a statement on April 14. Damage to the U.S. rating from the agencies is unlikely, S&P said then. In March, the Federal Reserve agreed to take on $30 billion of hard-to-trade assets of Bear Stearns Cos. to help secure its takeover by JPMorgan Chase & Co.

Fannie Mae and Freddie Mac would have to post pretax losses and writedowns of about $77 billion before the U.S. would be compelled to start a rescue, according to estimates by Fox-Pitt Kelton and Friedman, Billings, Ramsey & Co. analysts. The government-chartered, publicly traded companies have already raised $20 billion to cover losses amid the highest delinquency rates in at least 29 years.

``The notion that bringing the government-sponsored enterprises onto the federal balance sheet would `raise government debt by $5.3 trillion' and thereby sharply worsen the U.S. government's creditworthiness is misleading,'' Jan Hatzius, an economist at Goldman Sachs Group Inc. in New York, wrote in a note today. ``The government would have to cover any GSE losses, but this would be a much, much smaller number under any reasonable set of assumptions.''

-- With reporting by Brendan Murray and Dawn Kopecki in Washington and Elizabeth Stanton in New York. Editor: Dave Liedtka, Dennis Fitzgerald

To contact the reporter on this story: Lester Pimentel in New York at lpimentel1@bloomberg.net

Last Updated: July 11, 2008 15:39 EDT