US Airways and American Airlines will fight the US Justice Department's move to block their merger.
The Justice Department has filed a lawsuit to block the proposed merger between the two airlines, which would create the world's largest airline.
The 11 billion dollars deal was backed by a federal judge in March and has been approved by the European Union.
The complaint said customers would see a price rise as the merger would 'substantially lessen competition' in the domestic market.
The airlines rejected Justice Department arguments that the merger would limit choices for passengers, increase fare prices and reduce service.
They said they intend to mount a vigorous and strong defence of the merger, after it already gained the support of shareholders, creditors and European anti-trust regulators.
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, the report added.
