Shares of Spirit Airlines Inc. /zigman2/quotes/205782179/composite SAVE +1.65% rallied 3.0% in morning trading Tuesday, after the discount air carrier responded to JetBlue Airways Corp.'s /zigman2/quotes/207639051/composite JBLU +1.70% increased, unsolicited buyout bid by saying it was sticking to its merger agreement with Frontier Group Holdings Inc. /zigman2/quotes/211317788/composite ULCC +2.81% , the parent of Frontier Airlines. "The latest offer from JetBlue does nothing to address our Board's serious concerns that a combination with them would not receive regulatory approval," Spirit Chief Executive Ted Christie said. "While we have engaged with JetBlue for weeks and provided them a level playing field on which they could make their best offer, unfortunately they have now turned to scurrilous rhetoric instead of a substantive improvement in their offer." Christie's comment follows a statement from Frontier, in which the company said JetBlue was "not telling you the truth" about likely regulator concerns over antitrust issues. Meanwhile, shares of Frontier climbed 3.9% and of JetBlue advanced 3.0% in morning trading Tuesday, while the U.S. Global Jets ETF /zigman2/quotes/207744796/composite JETS +1.34% surged 3.1% and the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.39% inched up 0.1%.