American Express ( AXP) warned of a weakening U.S. economy and said it will record a $440 million pretax charge in the fourth quarter to reflect increased loan delinquencies.The news from the New York credit card giant is just the latest sign that problems in the subprime mortgage markets are spreading to other consumer credit classes and hurting spending. American Express said Thursday that it saw a slowdown of its card member spending to growth of about 13% in December, compared with 16% for the entire fourth quarter. The company also estimates that delinquencies in its managed U.S. lending portfolio increased to 3.2% in the fourth quarter from 2.9% in the third. The write-off rate increased to 4.3% to 3.7%. "While overall Cardmember spending continued to be relatively strong and we benefited from a focus on the affluent sector of the market, we did see some negative credit trends among U.S. consumers during December, particularly in California, Florida and other parts of the country most affected by the housing downturn," said Chairman and CEO Kenneth Chenault. The company's $440 million charge -- equivalent to $275 million on an after-tax basis -- will raise worldwide lending reserves to 100% of past-due loans and increase reserves related to the charge card portfolio. American Express now estimates fourth-quarter earnings from continuing operations at 70 cents to 73 cents a share, the company said. Analysts, on average, projected earnings of 87 cents a share for the period, according to Thomson Financial.