Ratings agency Moody's Investors Service upheld the United States' triple-A credit rating and stable outlook on Wednesday, but it noted potential longer-term threats to the rating.

Moody's, which has never changed the United States' triple-A rating, said the rating was supported by factors including a strong record of gross domestic product growth and the status of the U.S. dollar and Treasurys as the global reserve currency and bond market benchmark.

The continued stable outlook signals the rating is not likely to change over the next 12-18 months.

The ratings agency said that a downgrade could occur toward the end of the decade or into the 2020s, however, if fiscal policy remained unchanged and U.S. budget deficits and the debt ratios increase.

The agency cited spending on social programs as a concern.

Moody's said the U.S. government's debt to GDP ratio was stabilizing and noted that the United States remained on track to post its sixth straight year of expansion. Moody's also noted that U.S. growth, on a relative basis, compared more favorably with other triple-A rated economies than it did a year ago.