CHICAGO, July 10 -- United Airlines has called back 600 flight attendants who earlier took voluntary layoffs, and company officials said they plan to invite 851 more to return to work by late fall.

The first group is to return to work by Aug. 9, with the other to be recalled by the end of November. The airline, which is in Chapter 11 bankruptcy protection, will have invited back all flight attendants who agreed to take layoffs by December, United spokesman Jeff Green said. The airline announced the move Friday.

More flight attendants are needed partly because United planes are flying closer to capacity because of increased passenger demand, Green said. Last week, United reported that its planes flew 88.1 percent full in June, representing a record monthly load factor.

While the flight attendants union welcomed the recall, it also said the need for more flight attendants is because of others leaving over discontent about a round of pay and benefit cuts implemented by the carrier as it tries to emerge from Chapter 11 bankruptcy.

"We've got flight attendants leaving, and they need people to fill those jobs," said Sara Nelson Dela Cruz, a spokeswoman for the Association of Flight Attendants. "They're leaving because this isn't the same job it once was."

United's flight attendants are threatening to strike after the Elk Grove Village-based carrier, a unit of UAL Corp., formally turned over their pension plan to the federal Pension Benefit Guaranty Corp. A bankruptcy judge in May approved United's plan to unload its plans on the PBGC.

The flight attendants union said the termination violates its labor contract and triggered its legal right to strike. It has threatened to stage random, unannounced strikes.

With demand up, United says more flight attendants are needed because its planes are flying closer to capacity.