The ratings agency Fitch has warned that it may downgrade America's triple-A credit rating unless action was taken to reduce federal debt.

Fitch said the country's rising debt burden was not consistent with maintaining its top rating, but said it would probably not make a decision before 2013.

Last month, Fitch changed its US credit rating outlook to negative from stable, citing the failure of a special congressional committee to agree on at least $1.2tn in deficit-reduction measures.

"Federal debt will rise in the absence of expenditure and tax reforms that would address the challenges of rising health and social security spending as the population ages," Fitch said in a statement on Wednesday.

"The high and rising federal and general government debt burden is not consistent with the US retaining its 'AAA' status despite its other fundamental sovereign credit strengths," the ratings agency said.

Despite Fitch's warning, US debt is being valued at levels that suggest it deserves to keep its AAA status. In the bond market, 10-year Treasuries are trading near their record high levels, pushing down the yield on the bonds to just 1.97%.

Germany's 10-year bonds have a yield of 1.93%, with UK gilts have a yield of 2.06%. Both are AAA-rated nations. Ten-year debt issued by France, whose AAA rating looks rather shaky, is trading at a yield of 3.09%, – 50% higher than America's.

In its latest new fiscal projection, Fitch said at least $3.5tn of additional deficit reduction measures will be required to stabilize the federal debt held by the public at around 90 percent of gross domestic product in the latter half of the current decade.

Fitch, when it lowered its outlook to negative, had said it was giving the US government until 2013 to come up with a "credible plan" to tackle its ballooning budget deficit or risk a downgrade from the AAA status.

"A key task of an incoming Congress and administration in 2013 is to formulate a credible plan to reduce the budget deficit and stabilize the federal debt burden. Without such a strategy, the sovereign rating will likely be lowered by the end of 2013," Fitch reiterated.

Fitch's threat did not cause a panic in New York. The Dow Jones industrial average ended four points higher at 12,107, having been in negative territory for most of the day.

Rival ratings agency Standard & Poor's cut its credit rating on the United States to AA-plus from AAA in August, citing concerns over the government's budget deficit and rising debt burden as well as the political gridlock that nearly led to a default.

In November, Moody's warned that its top level AAA credit rating for the United States could be in jeopardy if lawmakers were to backtrack on $1.2tn automatic deficit cuts that are set to be made over 10 years.

The plan for automatic cuts was triggered after the special congressional committee failed to reach an agreement on deficit reduction. Moody's said any pullback from the agreed automatic cuts to take effect starting in 2013 could prompt it to take action.