Moody's Investors Services said it had initiated the review because of "the rising possibility" that Congress will fail to raise the debt ceiling by Aug. 2 -- something that could lead to a U.S. default on its debt.
If the debt ceiling isn't raised by then, the Treasury Department says it will no longer be able to pay all the country's bills in full and on time without being allowed to borrow new money. (Read: Debt ceiling FAQ)
"Moody's considers the probability of a default on interest payments to be low but no longer to be de minimis," Moody's said in a statement.
The United States enjoys its AAA rating in part for having always stood behind its debt and paid its bills on time. As a result, U.S. Treasury bonds are considered the world's safe-haven investment.
The Treasury Department issued an immediate response Wednesday.
"Moody's assessment is a timely reminder of the need for Congress to move quickly to avoid defaulting on the country's obligations and agree upon a substantial deficit reduction package," Treasury official Jeffrey A. Goldstein said in a statement.
In the still unlikely event the United States would default on any of its interest payments to bondholders, Moody's said it expected the default to be short-lived and the loss to bondholders "minimal or non-existent."
But, the agency added, a default "would fundamentally alter Moody's assessment of the timeliness of future payments." Translation: The United States would be downgraded to AA status. (read more)