The two carriers said on Friday they agreed to share confidential information, work with bankrupt AMR's creditors committee, and not talk to third parties about any terms of their possible combination.
American has also signed non-disclosure agreements with other parties, according to a memo the company sent to managers on Friday. It did not identify the other parties, and a spokesman for AMR declined to elaborate.
Many analysts consider US Airways to be the leading contender for AMR, which filed for Chapter 11 bankruptcy protection in November. Combined, US Airways and AMR had $37 billion in revenue in 2011, about equal with United Continental, the world's biggest carrier.
American Airlines initially said it would prefer to emerge from bankruptcy on a standalone basis, but has grudgingly responded to pressure from creditors to consider a merger as its best option for competing with United Continental and Delta Air Lines Inc (DAL.N), which are themselves the products of mergers.
A source familiar with the situation said the non-disclosure agreement is fairly restrictive as it prevents US Airways from discussing specific terms of a combination with third parties including labor unions and private equity firms that might be interested in a stake, or other airlines.
US Airways, which declined to comment beyond the news release, has been aggressively campaigning for a merger with AMR. Earlier this year, US Airways won the backing of three unions that represent American ramp workers, pilots and flight attendants by promising to preserve thousands of jobs post-merger.