Moody’s Investors Service says its triple-A rating for the United States could be threatened by the government’s exploding debt burden.

The credit rating agency made the same assessment for the United Kingdom and dropped both countries into the middle classification of its triple-A rating: “resilient.”

Top countries in the rating, including Canada, France and Germany, rank as “resistant.” The bottom triple-A classification, which includes Spain, is “vulnerable.”

"Moody's believes that the U.K. and U.S., the two countries that have lost altitude in the Aaa space, continue to warrant the ‘resilient’ characterization,” the agency said in a statement.

“However, to retain their ‘resilient’ status, the U.K. and U.S. will need to severely adjust their fiscal policies, even in the unlikely event of a vigorous rebound in their economies."

The U.S. budget deficit soared to $1.42 trillion in the year ended Sept. 30, reaching a post-World War II record of 10 percent of GDP.

The Congressional Budget Office forecasts that government debt will jump to 61 percent of GDP this year.

To be sure, the Moody’s report wasn’t completely negative on the U.S.

"Once there is a consensus on the need to address fiscal imbalances, the country continues to have an exceptional relative ability to grow out of its debt,” Pierre Cailleteau, managing director of Moody's Sovereign Risk Group, said in the statement.

“Its capacity to cut spending and/or raise taxes also remains significant."

Under Moody's most pessimistic scenario, the U.S. would lose its top rating in 2013 if the economy remains sluggish, interest rates appreciate, the deficit continues to balloon and banks that received bailouts are unable to pay the government back.

In contrast to several years ago, "the question of a potential downgrade of the U.S. is not inconceivable," Cailleteau said.

"In a world that has lost its compass a bit, people want to understand what happens to risk-free assets."

Many experts agree with Moody’s assessment.

“There has been a huge increase in debt-to-gross-domestic- product ratios as a result of the crisis,” David Keeble, head of fixed-income strategy in London for investment bank Calyon, told Bloomberg. “It’s right that there should be a lot of attention and pressure on these numbers.”

Bill Gross, who manages the world’s biggest bond fund for PIMCO Advisors, has predicted the U.S. ultimately will lose its triple-A rating thanks to the debt explosion.

“That’s the trend. It’s certainly nothing that’s going to happen overnight,” he told Bloomberg in May.