FORT WORTH, Texas (AP) — American Airlines flight attendants are asking federal officials for the right to go on strike, possibly before the end of the Christmas and New Year's travel rush, but American said there was "no possibility" of a walkout over the holidays.

Leaders of the flight attendants' union say they are frustrated with the lack of progress in negotiations over a new contract for workers who have not seen raises since 2019.

The Association of Professional Flight Attendants petitioned the National Mediation Board on Monday to declare the negotiations deadlocked and give the union permission to strike after a 30-day "cooling-off period."

Meanwhile, pilots at Southwest opened a "strike center" in Dallas this week. Officials with the Southwest Airlines Pilots Association say they too will ask for the right to strike if they don't have a contract deal with the airline in the next few days.

A digital clock on the wall at the pilots' union headquarters ticked down toward a potential strike on Dec. 29.

It is far from certain that either union will go on strike, however. Federal law makes it very hard for airline workers to walk off the job or for carriers to lock out workers.

Strikes and lockouts are legal only if federal mediators take the rare step of declaring that negotiations are at an impasse and that either side may resort to "self help." Even then, the president or Congress can block a strike that might hurt the economy.

The last strike by a U.S. airline union took place in 2010, involving pilots at Spirit Airlines.

American disputed the flight attendants' union claim that negotiations are stalemated. In a statement, the airline said that for months it has been offering the union an "industry-leading economic proposal," and progress continues on other contract items.

The airline, which is based in Fort Worth, said it is ready to continue working with the union and the National Mediation Board to reach an agreement.

American added that there is "no possibility" of a strike over Thanksgiving or the December holidays.

The flight attendants' union is asking American for immediate raises of 35% and then annual increases of 6% under a 3-year deal. American is offering 11% upfront but says it's 18% including higher pay during the time that passengers board planes, followed by annual increases of 2%. The union also wants bigger 401(k) contributions and increased rest time.

American's pilots recently won raises of more than 40% over four years.

"We definitely don't feel any equality here," said Erik Harris, treasurer of the flight attendants' union. "How come the pilots have gotten their deal and we haven't?"

Because of pattern bargaining, Southwest pilots are likely to wind up with raises like those approved for American Airlines pilots. The union at Southwest is asking for slightly higher pay than Boeing 737 pilots at other airlines, arguing that Southwest uses its planes — and pilots — longer per day on average.

A major stumbling block at Southwest is over pilot scheduling. The union wants Southwest to pay pilots a premium to operate flights that lack a crew instead of staffing those flights with pilots who are on reserve, or hold.

The Southwest pilots' union has already tried and failed once this year to get permission to strike. The union asked federal officials in June to release the group from mediation, but mediators refused. Another bargaining session is scheduled for the last week in November, but none after that.

"Nobody here wants to strike," said Tom Nekouei, the union's second vice president, "but we either need a deal by the 30th or we need to go down this path."

Dallas-based Southwest issued a statement saying that negotiations were continuing and it will work for a contract "that rewards our pilots and places them competitively in the industry."

Leaders of both the American flight attendants and Southwest pilots say they are encouraged by advances made by other unions this year.

The United Auto Workers won rich new contracts after a six-week strike, and screen and TV writers and actors got better compensation for streaming content and other concessions after strikes that crippled Hollywood for months. The Teamsters won hefty pay raises for more than 300,000 United Parcel Service workers by threatening to strike. Union organizing is on the rise.

On Thursday, several dozen American Airlines flight attendants picketed along a thoroughfare outside company headquarters in Texas, some carrying signs reading, "Ready to strike." Drivers in everything from sedans to gravel haulers honked in support.

"This gives me hope," said Harris, the union official, "but also seeing what's happening out there in labor worldwide is giving us all hope."