NEW YORK (CNN/Money) - US Airways, the nation's sixth-largest commercial airline, is expected to file for about $1 billion in loan guarantees from the federal government in an attempt keep its operations afloat, sources close to the carrier said Thursday.

The sources told CNN/Money that the airline is entitled to at least $750 million under the Air Transportation Stabilization Board (ATSB) standard, under which the federal agency distributes money based on market share.

But David Castelveter, a US Airways spokesman, said the company does not want to discuss how much it will request through the ATSB program. "We are working on the plan, and we haven't affirmed a date on the application," he said.

If US Airways (U: up $0.29 to $3.21, Research, Estimates), which has the highest fixed costs of any major airline, does file an application for a loan guarantee ahead of the June 28 deadline, it would become the second major domestic carrier to do so.

America West Airlines' parent America West Holdings Corp. (AWA: down $0.02 to $3.54, Research, Estimates) secured $429 million in loans backed by $380 million in government guarantees in mid-January, and said it dug up an additional $600 million in concessions, financing, and other assistance.

As a requirement for the loans, however, the government now holds warrants for up to one-third of No. 8 U.S. air carrier America West's common stock, and it extracted a commitment from the airline that it will control labor costs.

Arlington, Va.-based US Airways's business was severely damaged following the Sept. 11 attacks, due in part to the temporary closure of Washington, D.C.'s Ronald Reagan Washington National Airport, from which it currently runs more than 150 flights daily.

US Airways eliminated 111 aircraft, including three entire fleets of older planes since the attacks, and furloughed more than 11,000 workers to save on expenses. But it still posted a $2.1 billion loss last year.

Travelers on the East Coast, where US Airways runs its greatest concentration of flights, were more reluctant to fly after Sept. 11, and its market share has been whittled away by competitors running larger fleets of regional jets.  

--from staff and wire reports