WASHINGTON — Fitch Ratings said Tuesday that it would keep its rating on long-term U.S. debt at the highest grade, AAA, and said the outlook remained stable. But it warned that it could lower the outlook to negative if Congress fails to trim future deficits.

Fitch's rating was the best given by the three major credit rating agencies. Last week, Standard & Poor's set off a maelstrom in the stock market after it downgraded long-term U.S. debt to the second-highest level, AA-plus, for the first time. The third agency, Moody's Investors Service, still lists the U.S. debt at AAA but says its outlook is negative.

The Obama administration welcomed Fitch's announcement. Presidential spokesman Jay Carney told reporters on Air Force One that Fitch's rating was in line with the administration's view.

Treasury spokesman Anthony Coley added that Fitch's report “underscores the importance of Congress taking additional actions to address our long-term fiscal challenges.”