FORT WORTH, Texas (AP) _ The union representing American Airlines' flight attendants has filed suit over the company's plan to use foreign nationals on Latin American routes newly acquired from Eastern Airlines.

In a lawsuit filed Thursday in Miami, the Association of Professional Flight Attendants claims the plan violates American's contract with the 14,500-member union.

Earlier this week, American asked a Fort Worth federal judge to rule that the Railway Labor Act does not apply to flight attendants outside the United States and international flights to and from U.S. gateway cities.

American, the nation's largest airline, has agreed to pay Eastern $335 million for its routes between Miami and 15 Latin American nations. The sale is a key part of Eastern's reorganization plan.

American plans to hire all of Eastern's employees outside the United States, including flight attendants who are not U.S. citizens and who apparently will not be members of the airline's independent flight attendants' union.

The union also sought a preliminary injunction barring American from going ahead with its plans until the matter is put to arbitration. The Fort Worth- based airline is to begin serving some of the routes by July 1 and to serve them all by late August.

''What is at stake here is not only international flying in and out of Latin America to U.S. cities such as Miami, but international flying throughout the world,'' said Cheryle Leon, president of the flight attendants union.

Ms. Leon said the union had made an offer that would have allowed American to employ temporarily the foreign flight attendants coming from Eastern, but American rejected the offer.

Maureen Moore, an American attorney, said the airline has no choice but to hire those Latin American attendants now flying with Eastern because it agreed to do so as part of the acquisition.

Eastern had been held to the same obligation when it acquired those routes from the old Braniff International Airways in 1982, shortly before Braniff filed a Chapter 11 bankruptcy petition and ceased operating for nearly two years.

Ms. Moore said the flight attendants' union does not have a clause in its contract defining the extent of union coverage. And since American contends that the Railway Labor Act does not apply outside the United States, the airline believes the APFA has no right to represent citizens of foreign nations who work only on American international flights.

''Besides, some of these people are in their own unions,'' Ms. Moore said.