FORT WORTH, Texas (AP) _ The union representing American Airlines flight attendants has filed a federal lawsuit that seeks to stop the airline from firing attendants who walk out if a strike is called over the Christmas holidays.

The lawsuit, filed Wednesday by the Assocation of Professional Flight Attendants, also demands the airline stop calling a possible strike illegal.

The union, which represents 12,000 flight attendants at American, has threatened to strike over the holidays if a negotiating session scheduled for Friday proves fruitless.

The flight attendants have been working without a contract since April 24, when a cooling-off period between their union and the airline expired. American at that time unilaterally imposed the terms of its last contract proposal.

The suit asks that American be required to notify union members in writing that the strike is legal and that they won't be fired for engaging in a walkout.

The lawsuit also asks that American publically disavow, for seven consecutive days, previous statments about the illegality of a strike by attendants.

American spokesman Al Becker said the company stands by its statement last week in which it indicated the company regards a strike by attendants at this time as illegal.

American said it would permanently replace any attendants who strike and that it intends to keep its published schedule.

The airline has contended that a strike would be legal only during a ''reasonable time'' after the cooling off period, which extends 30 days after both sides declare contract talks have reached an impasse, under terms of the Railway Labor Act.

The union contends the strike would be legal, regardless of how much time has passed.