CHARLOTTE, N.C. ( TheStreet) -- A new report by advocacy groups said a potential merger between US Airways ( LCC) and American ( AAMRQ.PK) could substantially reduce airline competition, but US Airways President Scott Kirby called a merger "pro-consumer." US Airways, the fifth-largest U.S. airline, wants to merge with bankrupt American, the fourth largest, producing the largest carrier with an approximate market share around 20%. For its part, American said it wants to reorganize as a standalone carrier during its bankruptcy, then determine whether a merger makes sense. The report, released by the American Antitrust Institute and the Business Travel Coalition, said a merger would pose problems but stopped short of recommending that Department of Justice antitrust regulators reject it. US Airways has had preliminary conversations about a merger with officials from the Justice Department's antitrust division. Kirby said in an interview that a merger would be pro-consumer because "it would create an airline that can compete with United ( LCC) and Delta ( LCC) which neither US Airways nor American can do. Today, you have two (U.S.) airlines that serve the frequent flier who travels globally: This merger would create a third competitor." Told that the report identifies a half dozen markets where no competitors would remain after a merger, Kirby responded, "Six is a pretty small number. "Look at a route map," he said. "We have very complimentary and non-overlapping routes out of the hundreds of markets we serve." A half dozen markets would lose non-stop competition, the report said. The markets are Charlotte/Dallas; Charlotte/Miami; Philadelphia/Dallas; Philadelphia/Miami; Washington National/Nashville and Phoenix/Dallas. (Since the report was prepared, Spirit ( SAVE) has said it will fly Phoenix/Dallas starting in 2013.) The report raised several other questions about a potential merger. Among them: -- It challenged the long-standing assumption that low-fare carriers would provide sufficient fare competition to the legacy carriers. "This consolidation would occur against an industry backdrop marked by a dwindling fringe of low-cost carriers and growing questions as to whether legacy look-alike Southwest ( LUV) exerts any significant competitive discipline in the industry," the report said.