CHICAGO (CNN) - Mechanics for United Airlines said Thursday they rejected their part of a $1.5 billion package of wage concessions designed to keep the company out of bankruptcy and qualify it for nearly $2 billion in federal loan guarantees.

United President and CEO Glenn Tilton said he was disappointed in the mechanics' decision, but would work to quickly bring another contract proposal before the group.

United's 13,000 Mechanic and Related employees, who are members of the International Association of Machinists and Aerospace Workers (IAM), rejected the package, with 57 percent voting against it, IAM noted in a statement released Thursday.

"Clearly, we're disappointed that our mechanic and related employees, represented by IAM District 141M, did not approve the tentative agreement with United," Tilton said. "Nevertheless, we remain fully committed to the goals of the United Airlines Union Coalition in achieving labor cost savings that will enable us to secure federally backed loans."

Two other groups of United employees who are IAM members and who represent 24,500 workers -- including customer-service agents, baggage handlers and reservation agents -- approved their portion of the wage-concession package. Ramp & Stores, Food Service and Security Guards ratified their agreements with 63.4 percent of voters approving the accords. The Public Contact Employee (PCE) group, meanwhile, also accepted their agreement, with 79.2 percent endorsing the contract, IAM said.

"Each employee measured the costs and benefits of participating in United's recovery plan," said Scotty Ford, IAM District 141-M president, in the association's statement. "In the end, some thought the risk was worth taking, and others felt they had sacrificed enough. We respect both decisions and this organization will aggressively represent their common interests as this extraordinary situation unfolds."

Saturday a vote by the rank and file of the Association of Flight Attendants is scheduled. That labor deal would save the airline about $400 million. The pilots have already ratified a labor deal designed to save the airline $2.2 billion.

United recently secured $5.2 billion in wage cuts from its employees, including five separate unions, as part of a financial recovery plan put before the Air Transportation Stabilization Board (ATSB). That is a new federal agency created after the Sept. 11 attacks and charged with doling out up to $10 billion in loan guarantees.

Elk Grove Village, Ill.-based United, a unit of UAL Corp. (UAL: up $0.12 to $3.63, Research, Estimates), has asked the agency to back $1.8 billion of a $2 billion loan. It has met with staffers every week after the board said more labor wage concessions were needed than what the airline originally outlined.

Industry experts say a decision from the ATSB will determine the near-term fate of United as it tries to avoid restructuring through the courts.

The airline has $375 million in bond payments due Monday, and it is not expected to make that payment without some positive sign on the federal loan guarantee.

United has been burning off more than $7 million in cash a day, said Philip Baggaley, the managing director for airlines and aerospace companies at credit rating agency Standard & Poor's. By doing so, it is quickly eating through the $1.7 billion it had in reserve at the start of the fourth quarter.  

-- Reuters contributed to this article.