The rating agency said the United States’ embrace of fiscal stimulus rather than debt reduction creates “a small but increasing likelihood that markets will demand a higher risk premium on government debt, in sharp contrast to the safe-haven status that the U.S. Treasury bond has long enjoyed.”

Moody’s noted in its quarterly Aaa Sovereign Monitor report the contrast between America’s spend-first stance and the austerity policies being pursued with varying levels of vigor in the three other triple-A rated major Western economies – the U.K., Germany and France.

The risk in Europe, Moody’s said, is that deep spending cutbacks will weigh so heavily on growth that even thrifty policies won’t make a dent in massive debt loads. It said strong recent German economic growth reduces those concerns somewhat, but “the game is not yet over.”

In the United States, the rating agency said, the risks are more acute. It suggested the failure of the White House and Congress to confront our fiscal problems could spark a bond market panic much like the one former Fed chief Alan Greenspan has spent recent months warning of.

The continued high level of deficits and upward debt trajectory could cause borrowing costs to rise more than now expected, making progress in reducing deficits more difficult in the future.

The rating agency said it views borrowing costs in the different rich countries in part as “a test of how fiscal policy should be managed in the wake of a major financial crisis,” though it conceded the test isn’t perfect thanks to complicating factors such as varying monetary policy stances and the dollar’s global reserve status.

Moody’s comments come as investors grapple with how rich nations will dig their way out of debts they took on during the financial crisis, and pay for the promises they have made to provide health and retirement benefits to their citizens.

Government bond yields remain near historic lows, but the yield on the 10-year U.S. Treasury note has risen by a point or so (see chart, right) since it bottomed out in October at the tail end of the market’s obsession with deflation. The rise hasn’t come only in the United States, which says investors are worrying more about fiscal problems in all the big, rich nations.

Despite varying economic and political conditions, the U.S., the United Kingdom, Germany and France all face “dramatic increases under their existing policy commitments arising from ageing-related pension and healthcare subsidies. These future costs must be brought under control if these countries are to maintain long-term stability in their debt burden credit metrics.”