Senator Richard Blumenthal, a Democrat who has been outspoken on antitrust issues, expressed concern to the Transportation Department on Friday that a partnership between American Airlines (AAL.O) and JetBlue Airways (JBLU.O) would lead to higher airfares.

The codeshare agreement allows American and JetBlue to sell each other's flights in their New York-area and Boston networks and link frequent flyer programs, in a move aimed at giving them more muscle to compete with United Airlines (UAL.O) and Delta Air Lines (DAL.N) in the U.S.

"I write with grave concerns that the recent joint partnership between American Airlines and JetBlue Airways will lead to anticompetitive coordination at key air traffic hubs and result in the long-term inflation of airfares and related costs for airline passengers," Blumenthal said in a letter to U.S.

In a statement, JetBlue said the partnership allowed the two airlines to give Delta and United real competition.