The $21 trillion debt the U.S. has amassed on its balance sheet isn't weighing on the minds of credit rating agencies.

Moody's and Fitch in recent days have reaffirmed the nation's top-notch credit standing, reasoning that even with the massive pile of IOUs, the nation has sufficient resources to keep its standing.

"The affirmation of the US' Aaa rating reflects the US' exceptional economic strength, the very high strength of its institutions and its very low exposure to credit-related shocks given the unique and central roles of the US dollar and US Treasury bond market in the global financial system," Moody's analysts said in a report Wednesday afternoon.

Those relatively glowing remarks come even as the debt tally continues to rise.

Total public debt outstanding was at $21.06 trillion as of Tuesday, a 2.8 percent rise since the beginning of the year. Of that total, $15.34 trillion is owed by the public.

There have been multiple warnings lately about the surging level — the Congressional Budget Office said the U.S. would be running a $1 trillion budget deficit within the next couple of years, and Federal Reserve Chairman Jerome Powell has said repeatedly that the nation is on an unsustainable fiscal path.