They said the airline had not settled on its revamped structure nor provided much information about the low-fare operation that is the centerpiece of its plan.

Executives at AMR's American Airlines have also said they are studying whether to start a low-fare operation.

The market demand was there to make a no-frills, low-fare operation successful.

Now that United and Delta have waded into the battle with low-fare operations, the industry is wondering if and when the market leader, American, will do the same.

In some cases, they have introduced new service, like United's low-fare operation, Ted.

Last month, Delta began flights by Song, its low-fare operation focusing primarily on the East Coast, with fares from $79 to $299 one-way.

It warned that it would cut its price by a "significant" amount if Lufthansa failed to offload low-fare operation Bmibaby before any deal completes.

Sean Donohue, United's vice president in charge of low-fare operations, wants passengers to think of Ted as a friend, albeit one with wings and a tail.

The crop now includes Song, the low-fare operation started by Delta in April.

However, Delta said it would spare Song, its low-fare operation, whose demise had been widely rumored.