CHICAGO — A merger between United Airlines and US Airways would hit the smaller hub airports where the two operate hardest as the carriers would likely move to consolidate those bases, Moody’s Investors Service warned in a new report on the potential credit impact of a union between the two.

Reports have circulated this week that Chicago-based UAL Corp.’s United Airlines and Tempe, Ariz.-based US Airways Group Inc. have been in discussions since February on a possible merger that would create the second-largest air carrier behind Delta Air Lines. United is currently the third largest and US Airways is sixth largest.

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