US Airways and American Airlines say they will fight a US Justice Department's move to block their merger, which would create the world's largest carrier.
US Airways and American parent AMR Corp have rejected Justice Department arguments that the merger would limit choices for passengers, push up fare prices and reduce service.
They said in a statement they "intend to mount a vigorous and strong defence" of the $US11 billion merger, after it already gained the support of shareholders, creditors and European anti-trust regulators.
"We will mount a vigorous defence and pursue all legal options in order to achieve this merger and deliver the benefits of the new American to our customers and communities as soon as possible," they said.
"We believe that the DOJ is wrong in its assessment of our merger, they said, referring to the Justice Department.
"Integrating the complementary networks of American and US Airways to benefit passengers is the motivation for bringing these airlines together. Blocking this pro-competitive merger will deny customers access to a broader airline network that gives them more choices."
They also rejected the department's allegation that the merger was not critical to completing AMR's bankruptcy restructuring program.
"The widespread support from the employees and financial stakeholders of both airlines underscores the fact that this is the best path forward for both airlines and the customers and communities we serve."
In a letter to staff released to the media, US Airways chief executive Doug Parker stressed management's commitment to the merger, while delaying the original target closing date of the third quarter of this year.
"Other companies have found themselves in similar circumstances and gone on to successfully close their merger," he said.
"We are hopeful that the litigation will be successfully concluded and we will close the merger before year end."