As United and Continental continue to combine their operations, the latest casualty will be 688 flight attendants at United who will be furloughed in April despite a hiring push for the same position at Continental, which hired 485 last year.
The action stems from the inability of United and its union to agree on a unified contract for the combined airline.
"We have offered opportunities to flight attendants for both voluntary furloughs and job-sharing programs in order to mitigate involuntary furloughs, but these programs did not generate enough volunteers and we are faced with the difficult step of furloughing 688 flight attendants," United spokeswoman Megan McCarthy said Thursday in an email to CNBC.
(Read more: Optimism boosts business travel spending)
"We are disappointed that on three occasions the subsidiary United AFA rejected a voluntary crossover program that would have provided flying opportunities to hundreds of flight attendants that may otherwise be involuntarily furloughed," she said.
But the union disagrees with that characterization.
"Successful airlines do not lay off workers—they work with the union for solutions," Greg Davidowitch, the president of the Association of Flight Attendants-CWA for United, said in a statement issued late Wednesday.
"We continue to meet with management and offer creative solutions to an involuntary furlough; while also addressing the company's needs to mitigate an overage in manpower. They will try and create clever terms to justify their choice, and most certainly drop the blame on the union, but what it comes down to is simply the promise of United's merger not being realized," the statement read.
United's announcement "is a failure by management to recognize protections of our contract," said Davidowitch, who leads 25,000 flight attendants at United.
The furloughs may feel more like an actual layoff: There is no set date to return to work other than when the airline decides to hire again.
McCarthy said that last year the airline offered the Continental jobs to the United flight attendants, but the union declined any crossover. As a result, Continental hired 485 flight attendants last year and is hiring an additional few dozen language-qualified attendants this year.
Christopher Clarke, spokesman for the Association of Flight Attendants-CWA for United, said the union had accepted the crossover offer in November with the condition that no other furloughs would happen.
The airline did not respond to that stipulation, he said. In addition, about 300 of the United flight attendants have volunteered to take time off rather to avoid a furlough, but the airline has declined.
All this is taking place amid the backdrop of a larger contract negotiation.
"We're not close," Clarke said.
(Read more: As the mile gap widens, new flier loyalty tactics)
Continental isn't the only place with a help wanted sign.
Southwest said it added 600 flight attendants in 2013 and aims to bring on 750 more this year, said Melissa Ford, a spokeswoman for the carrier.
Those 750 positions were posted in December, she added, and "it took two hours and five minutes to receive 10,000 résumés."
At Southwest, Ford said, "the hiring is driven by variables such as attrition, increased vacation weeks, the fourth flight attendant required for 737-800 deliveries, and the AirTran aircraft conversion schedule. So not all 1,350 are new positions."
Before December, she said, 2011 was the last time the position was posted externally.
(Read more: Southwest pressed to break SeaWorld ties)
—By CNBC's Amy Langfield. Follow her on Twitter at @AmyLangfield.
Follow Road Warrior on Twitter at @CNBCtravel.