(Reuters) -JetBlue Airways and United Airlines on Thursday asked the U.S. Transportation Department to reject a complaint filed by Spirit Airlines over their proposed partnership.

United and JetBlue said in May their "Blue Sky" tie-up would allow travelers to book flights on both carriers' websites, while interchangeably earning and using points in their frequent flyer programs. Budget carrier Spirit said last month the deal would mean the smaller JetBlue "will become a de facto vassal of United." United and JetBlue rejected the complaint calling it a "fiction" and adding: "Blue Sky will maintain JetBlue's independence and enhance its competitiveness, ensuring that

JetBlue can continue to grow and compete as an independent carrier."