Regulators in Washington and in Europe are scrutinizing the deal based on antitrust concerns, The New York Times reported Saturday.
The concern is that the airline would be so large it would dictate prices and less competition would hurt consumers.
The Government Accountability Office, a bipartisan accounting research office and not a regulator, has concluded the merger would reduce competition in more markets than has occurred in previous airline mergers.
The GAO released its findings in June, the Times said.
American Airlines went into bankruptcy in November 2011, but based on its size a merger between AA and US Airways would give U.S. Airways shareholders 28 percent of the combined company, the Times said.
US Airways Chairman and Chief Executive Officer W. Douglas Parker said Friday at a shareholder meeting the combined businesses would operate 6,500 flights daily. He said customers and shareholders would benefit from a merger.