The routes in question serve 2.5 million passengers a year
American Airlines has hit back at a US Justice Department report that says its tie-up with British Airways and Iberia could lead to "competitive harm".
In papers filed this week, the Justice Department said fares between six transatlantic routes "could increase up to 15% under the proposed agreements".
The report advised removing some of the carriers' take-off and landing slots.
American Airlines said it disagreed with the report and believed the deal would be approved.
"American looks forward to rebutting the points made," the airline said.
British Airways also said it would "be making a robust response" to the Justice Department's comments.
The airlines want to operate as a joint business, called the Oneworld alliance, on flights between Europe and the US.
The Department of Justice wants the Department of Transportation to impose conditions to protect competition.
The report notes that "American and Iberia are the only current non-stop competitors between Miami and Madrid."
Fares between Boston and London, Chicago and London, Dallas and London, Miami and London, Miami and Madrid, and New York and London could be affected.
The routes in question serve 2.5 million passengers a year.