Ratings agency Fitch has reaffirmed the United States’ top credit rating, but downgraded the outlook to negative as it projected slow growth, political stalemate and rising levels of debt this decade.

The rating agency said it has less confidence in the US government’s ability to take the necessary steps to rein in the deficit.

Citing “still strong economic and credit fundamentals,” Fitch nonetheless said the recent failure of Congress to reach a short-term deficit cutting deal could delay more fundamental reforms.

A special congressional panel failed last week to reach agreement on $US1.2 trillion ($A1.22 trillion) in deficit cuts over the next decade.

The impasse triggered automatic cuts of the same amount, which are scheduled to kick in beginning in 2013.

Fitch added that there was “considerable uncertainty surrounding the economy’s potential output”.

Taken together, political failures and slower growth could result in a full-fledged downgrade.

“The negative outlook indicates a slightly greater than 50 per cent chance of a downgrade over a two-year horizon.”

Fitch said a key trigger would be the government’s failure to reach agreement in 2013 on a “credible deficit reduction plan” as the economy slows.

That, Fitch said “would likely result in a downgrade of the US sovereign rating.”

“The longer productive capacity remains idle and unemployment high, the greater the likelihood that the loss of output (and tax receipts) is greater than currently estimated.”

Fitch said that had “negative implications for the medium to long-term fiscal outlook.”

Fitch projected federal debt would rise to 90 per cent of GDP by the end of the decade.

“In Fitch’s opinion, such a level of government indebtedness would no longer be consistent with the US retaining its ‘AAA’ status despite its underlying strengths.”

Moody’s Investors Services and Standard & Poor’s also left their ratings unchanged last week. But Moody’s threatened to lower its rating if Congress backed off the automatic cuts.

S&P downgraded long-term US debt in August to the second-highest level, AA-plus, and switched its outlook to negative.

It was the first time the credit rating agency had lowered the nation’s AAA rating since granting it in 1917.

Fitch, Moody’s and S&P, the three major rating agencies, assess the creditworthiness of debt issued by countries, corporations and municipalities.

Ratings are based on the likelihood of default. The AAA rating is the highest available and signifies an extremely low likelihood of default.