The bill to raise the country's borrowing limit and prevent a possible U.S. debt default passed in Congress. But it not enough for the U.S. to maintain its coveted AAA debt rating, according to Fitch Ratings.

On Tuesday, Fitch said the agreement was an important first step but "not the end of the process." The rating agency wants to see a credible plan to reduce the budget deficit.

David Riley, managing director at Fitch, told The Associated Press: "There's more to be done in order to keep the rating in the medium-term."

Fitch expects to conclude its review of the U.S. sovereign rating by the end of August. As the debt deal currently stands, it is possible the U.S. debt rating could be downgraded at that time, Fitch said.