Ratings agency Moody's on Monday sounded a cautious note about the implementation of the United States' current debt plan, warning it would downgrade the country if its deficit-reduction measures are not "credible."

In his first comments after rival Standard & Poor's stripped the United States of its AAA rating on Friday, Moody's analyst Steven Hess repeated that the Aug. 2 plan to cut deficits by $2.1 trillion was positive for the U.S. credit standing, although not enough to keep its rating on a stable outlook.