U.S. banks face a "serious risk" that their creditworthiness will deteriorate if Europe's debt crisis deepens and spreads beyond the five most-troubled nations, Fitch Ratings said.

"Unless the euro zone debt crisis is resolved in a timely and orderly manner, the broad credit outlook for the U.S. banking industry could worsen," the New York-based rating company said yesterday in a statement. Even as U.S. banks have "manageable" exposure to stressed European markets, "further contagion poses a serious risk," Fitch said, without explaining what it meant by contagion.

The "exposures" of U.S. lenders to major European banks and the stressed nations of Greece, Ireland, Italy, Portugal and Spain, known as the GIIPS, are smaller than those to some of the continent's larger countries, Fitch said.

The six biggest U.S. banks -- JPMorgan Chase & Co. (JPM), Bank of America Corp. (BAC), Citigroup Inc. (C), Wells Fargo & Co. (WFC), Goldman Sachs Group Inc. and Morgan Stanley (MS) -- had $50 billion in risk tied to the GIIPS on Sept. 30, Fitch said. So-called cross-border outstandings to France for all except Wells Fargo were $188 billion, including $114 billion to French banks. Risk to Britain and its banks was $225 billion and $51 billion, respectively.

Europe's debt crisis has toppled four elected governments, with the last two, in Greece and Italy, falling last week. Italian bond yields remained at about 7 percent -- the threshold that led Greece, Portugal and Ireland to seek bailouts -- and shares of French banks, including BNP Paribas (BNP) SA and Societe Generale (GLE) SA, dropped amid concern they'll need more capital.

U.S. stocks slumped yesterday after the Fitch report was released. The Standard & Poor's 500 Index slid 1.7 percent and the 24-company KBW Bank Index declined 1.9 percent. U.S. index futures fell earlier today as Spanish and French borrowing costs rose.

The Fitch report is a worst-case scenario and is "oddly out of step" with the rating company's previous reports, analysts at HSBC Holdings Plc said today. U.S. banks may even benefit as investors shift money to the U.S. from Europe, HSBC said.

Investor demand for the relative safety of Treasuries during the European debt crisis has sent the difference between U.S. short-term yields and bank rates surging to levels not seen in more than two years.

The gap between the London interbank offered rate and the overnight index swap, or what traders expect the Federal Reserve's benchmark to be over the term of the contract, widened to 38 basis points today. It was the highest level since June 2009.

U.S. five-year swap spreads climbed to 45 basis points, the most since August 2009. Investors use swaps to exchange fixed and floating interest rates. The spread, the gap between the fixed component and the yield on similar-maturity Treasuries, is a measure of bank creditworthiness.