CHARLOTTE, N.C. ( TheStreet) -- Five labor unions at US Airways ( LCC) and American Airlines ( AAMRQ.PK) have signed onto a joint statement voicing support for the merger between the two carriers.

But the biggest union at US Airways, which represents about 14,000 mechanics and fleet service workers, stayed away. Leaders of the International Association of Machinists said they cannot back a merger until their contract is signed.

"IAM members at US Airways need and deserve contracts now," said IAM District 141 President Rich Delaney, in a prepared statement. "The machinists union will not allow US Airways to stall our members' contract negotiations while it devotes its attention to merging with American Airlines. The IAM is prepared to support this merger only if it provides real value to employees."

"The IAM has been in negotiations with US Airways to amend existing contracts for approximately two years," said IAM District 142 President Tom Higginbotham, in a prepared statement. "US Airways, however, has been more concerned with courting American Airlines than negotiating with its own employees."

In an interview on Thursday, US Airways President Scott Kirby said "all of the American Airlines unions and two at US Airways (support) the merger. We are in talks with the IAM and hope to have them on board too."

-- Written by Ted Reed in Charlotte.