Fitch Ratings has affirmed the United States' top-flight 'triple A' debt rating. Fitch says the world's largest economy remains strong and is supported by having the world's leading reserve currency. Fitch also noted that the country's 2.4 percent growth rate last year topped most other advanced economies. Fitch said US debt continues to rise but remains well within the tolerance of the 'AAA' rating. The credit rating agency added that the American economy remains one of the most productive, dynamic and technologically advanced in the world.