Still, US Airways said Tuesday it remained a "disciplined and determined bidder for Delta."

Doug Parker, US Airways' chairman and chief executive, said in a prepared statement that the combining of US Airways and Delta would create "$1.65 billion in annual synergies beyond the value that could be created by any stand-alone plan."

"These synergies come on top of the certainty of $4.0 billion in cash and the upside potential of 78.5 million shares of US Airways stock," Parker said. "These shared synergies will benefit all shareholders in the 'new' Delta. Factoring the synergy benefits into our offer, the current value of our proposal is significantly greater than the value of Delta's stand-alone plan."

US Airways has filed for bankruptcy twice. In 2005, it was acquired by America West.

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