The United States is one of the few countries where Congress sets a ceiling on government debt, which creates "periodic uncertainty" over the government's ability to meet its obligations, Moody's said in a report.
"We would reduce our assessment of event risk if the government changed its framework for managing government debt to lessen or eliminate that uncertainty," Moody's analyst Steven Hess wrote in the report.
The agency last week warned it would cut the United States' AAA credit rating if the government misses debt payments, increasing pressure on Republicans and the White House to come up with a budget agreement.
Moody's said it had always considered the risk of a U.S. debt default very low because Congress has regularly raised the debt ceiling during many decades, usually without controversy.
However, the current wide divisions between the House of Representatives and the Obama administration over the debt limit creates a high level of uncertainty and causes us to raise our assessment of event risk," Hess said.
Stepping further into the heated political debate about U.S. debt problems, Moody's suggested the government could look at other ways to limit debt.
It cited Chile, widely praised as Latin America's most fiscally-sound country, as an example.
"Elsewhere, the level of deficits is constrained by a 'fiscal rule,' which means the rise in debt is constrained though not technically limited," Moody's said, adding that such rule has been effective in Chile.