Moody's Investors Service affirmed the U.S.'s top rating for the second time in a week, rebutting Standard & Poor's after it stripped the world's largest economy of its top credit status.

The U.S. today retained its Aaa ranking with a negative outlook in part because the dollar's status as the main reserve currency allows it to support higher debt levels than other countries, Moody's said today in a report. Lawmakers last week took a "positive step" toward addressing the nation's record deficits, Steven Hess, the senior credit officer at Moody's in New York, said in a telephone interview.

"What we've said over the last few months is that we now see at least both parties having the same goal of deficit and debt reduction over the long term, even though more needs to be done," Hess said. "More important to us than how contentious the process is, does it produce results? What we're looking at is actual policies as opposed to the political debate."