A U.S. debt default could rock the global economy at a dangerously vulnerable time, with growth already slowing in the Asian juggernauts of China and India even as signs emerge that Europe is failing to prevent its own debt crisis from spreading into its biggest economies.

Although a bitter day of reckoning over the impasse in Washington did not materialize Monday — stocks dropped, but modestly, from New York to London to Tokyo — a tipping point could be approaching fast.

One truth has so far spared world markets from a massive, collective sell-off: Most big investors see it as unlikely that the United States will actually renege on its obligations with the holders of U.S. Treasury bonds.

But economists warn that a psychological barrier may be approaching in the coming days that could shatter faith in Treasurys, long seen as the world’s most stable investment. Crossing that threshold, they say, risks sparking the kind of panic that sent world markets into a tailspin three years ago.

“This would be bigger than September 2008,” Simon Johnson, economic analyst at the Peterson Institute in Washington, said of a possible U.S. default. “You could be talking about another world recession.”

In the event of even a limited default, the world economy would suddenly be plunged “back into uncharted territory,” said Thomas Mayer, chief economist at Deutsche Bank in Frankfurt.

“You’d have to presume a severe stock market sell-off globally at a time when there are already signs of a global slowdown,” he said. “The only nations that would not feel such an event are those not integrated in the global financial system. You’d have to be North Korea.”

For now, the markets are standing firm. U.S., Asian and European markets were all down Monday, but by relatively small margins. Asian markets edged slightly higher in morning trading Tuesday. “I think the markets are handling this in a very, very calm manner,” said Fred Dickson, chief investment strategist at D.A. Davidson in Portland, Ore.

But the International Monetary Fund warned Monday that may not last, calling for an urgent solution to the political impasse in Washington so the markets do not lose confidence in U.S. debt.

The IMF’s board warned that a default would “have significant global repercussions, given the central role of U.S. Treasury bonds in world financial markets.”

The timing of a U.S. debt crisis could not be worse.

Europe’s bid to resolve its own roiling debt problems appears to be faltering. Its leaders last week agreed to a sweeping second bailout for near-bankrupt Greece that will also impose losses on some investors, mostly big European banks, in what economists are calling a “limited” default.

Although the agreement was hailed as a breakthrough — initially calming fears that far larger indebted nations would catch Greece’s cold — a fresh cycle of worry hit the region on Monday.

It happened after Moody’s again slashed its rating on Greek debt deeper into junk bond territory and warned that Athens’ limited default might become a model for other troubled European nations, leading to renewed sell-offs of the bonds of Spain and Italy.

Analysts warned that creaking confidence in U.S. debt could heighten fears that even major nations in Europe cannot be trusted to pay their bills. Punctuating that point, Moody’s warned that relatively healthy European economies on the hook for big bailouts could see their own credit ratings erode, which potentially leaves nations such as France exposed.

In the event of a U.S. default, analysts warned of a possible spike in global borrowing costs for a bevy of industrialized nations as investors lose confidence in government debt. That would further damage wealthy nations just as growth is faltering and employment stagnating in Europe and the United States.

In Europe on Monday, there was a growing sense that the real threat to the global economy lies on the other side of the Atlantic. Officials expressed mounting disbelief and concern over the stalemate, with some openly sympathizing with President Obama.

“The biggest threat to the world financial system now comes from a few right-wing nutters in the American Congress rather than the euro zone,” Vince Cable, Britain’s minister for business affairs, said in unusually blunt comments late Sunday.

And yet, analysts said few major investors in Europe have seriously contemplated the impact of a possible U.S. default because they refuse to believe it is a possibility.

If Washington does run out of cash, many economists say the U.S. government would instead be forced to heap the pain onto American citizens, shutting down public offices or withholding Social Security and military paychecks. They would use the savings to pay foreign investors until a deal to raise the U.S. debt ceiling can be reached.

U.S. woes come as growth is slowing in what have been two major drivers of the global economy, China and India.

In China, the slowdown to an estimated — and still remarkable — 9 percent is actually an attempt to prevent the economy from overheating. But if an American default were to drive up U.S. interest rates and markedly slow the U.S. economy, it could corrode Chinese growth as American consumers cut back on buying Chinese-made goods.

Thus far, however, the fear most prevalent in China — the world’s largest holder of U.S. debt — is seeing the value of its investment portfolio decline if the credit ratings agencies punish America with a downgrade.

But China does not appear to have an alternative for its massive stock of foreign exchange, particularly with the euro zone still grappling with its own debt crisis.

In India, where the still-brisk growth of 7.8 percent in the first quarter of the year nevertheless amounted to the slowest pace since the end of 2009, the Central Bank warned Monday of external shocks.

“Globally,” the bank said, “the momentum of recovery appears to be stalling.”

Staff writers Keith B. Richburg in Shanghai, Rama Lakshmi in New Delhi and Cezary Podkul in Washington contributed to this report.