The airlines said they would work "in good faith" and "close collaboration" with American's unsecured-creditors committee. They noted there was "no assurance that a transaction will result."
American also signed a similar agreement with International Consolidated Airlines Group SA, owner of British Airways, Bloomberg News reported.
US Airways, the dominant airline in Philadelphia, has aggressively pursued a possible merger with American since soon after AMR Corp. filed for bankruptcy in November.
In a letter to US Airways employees Friday, CEO Doug Parker said he was "pleased to be working directly with American to study a potential merger."
"It does not mean we are merging - it simply means we have agreed to work together to discuss and analyze a potential merger," Parker wrote.
American, in a note to managers Friday, said that, as part of the process, "other parties have also signed confidentiality agreements, which permit for the confidential exchange of information and discussion between American and those parties." The airline did not identify them.
The memo repeated remarks made recently by CEO Thomas Horton that American has gained "clarity" on its revenue outlook and cost structure and was "now looking at other strategic options that could make the new American even stronger."
American will move through the evaluation process with the goal "to create the highest value for our creditors and the best outcome for our people," the memo said.
A merger between AMR Corp., American's parent, and US Airways would create an airline about the same size as the world's largest, United Continental Holdings, and bigger than No. 2 Delta Air Lines.
"AMR has been dragged kicking and screaming to the table," bond analyst Vicki Bryan, with Gimme Credit L.L.C., said in a client note. "AMR's stakeholders are becoming universally convinced that a merger presents the best chance" to compete with larger Delta and United, "as well as global behemoths emerging overseas."
American's flight-attendant union reiterated support Friday for a merger with US Airways. "Combining our networks will allow American Airlines to grow and compete with our industry's dominant global carriers."
Contact Linda Loyd at 215-854-2831