US Airways and American Airlines parent company AMR Corp. are now in formal negotiations on a possible merger.

The U.S. carriers today jointly announced that they have entered a non-disclosure agreement (NDA) “under which both companies have agreed to exchange certain confidential information and, in close collaboration with AMR’s Unsecured Creditors Committee, to work in good faith to evaluate a potential combination.”

The NDA was proposed several weeks ago by AMR, but it was only in the past week that US Airways started talking about possibly agreeing to its terms. US Airways for months has vocally promoted its own merger plan with AMR, although that proposal initially was rejected by the Fort Worth-based operator.

US Airways CEO Doug Parker sent a message to employees in which he said, “We are pleased to be working directly with American to study a potential merger, and we consider this very good news. It does not mean we are merging—it simply means we have agreed to work together to discuss and analyze a potential merger.”

Under the NDA, American and US Airways officials have agreed not to disclose publicly any details of the negotiations until they reach an agreement or terminate the negotiations.

Pilots and flight attendants unions at the respective carriers already have launched their own formal discussions on a possible merger of AMR and US Airways, and US Airways months ago negotiated contract terms with AMR’s unions should its merger plan be successful.