With attention focused on sovereign-debt worries in Europe, two major credit-rating firms reminded investors again that the U.S. has debt problems of its own.

Investors bought Treasury debt nonetheless, ignoring the comments, which echoed prior statements by the companies and may still be months or years away from having any practical meaning.

"The warning on the U.S. rating is well-founded," said Brian Yelvington, chief fixed-income strategist at Knight Capital. "However, it will probably fall on deaf ears until the peripheral Europe story plays out."

Moody's Investors Service said in a report on Thursday that the U.S. will need to reverse the expansion of its debt if it hopes to keep its "Aaa" rating.

"We have become increasingly clear about the fact that if there are not offsetting measures to reverse the deterioration in negative fundamentals in the U.S., the likelihood of a negative outlook over the next two years will increase," Sarah Carlson, senior analyst at Moody's, said.