Moody's Investors Service and Fitch Ratings affirmed their AAA credit ratings for the U.S. while warning that downgrades were possible if lawmakers fail to enact debt reduction measures and the economy weakens.

The outlook for the U.S. grade is now negative, Moody's said in a statement yesterday after President Barack Obama signed into law a plan to lift the nation's borrowing limit and cut spending following months of wrangling between Democratic leaders and Republican lawmakers.

The compromise "is a positive step toward reducing the future path of the deficit and the debt levels," Steven Hess, senior credit officer at Moody's in New York, said in a telephone interview yesterday. "We do think more needs to be done to ensure a reduction in the debt to GDP ratio, for example, going forward."