The International Association of Machinists and Aerospace Workers said on Thursday that it had moved to seek a union election at Delta Air Lines Inc to resolve representation for 12,700 employees at the carrier and its Northwest subsidiary.

The union said it had asked the National Mediation Board to declare that Delta and Northwest were operating as a single carrier, a necessary step in the process that would lead to a union election for fleet service, flight simulator and plant protection employees.

Delta acquired Northwest Airlines last year.

Machinists union spokesman Joseph Tiberi said about 5,000 of the 12,700 staff it is currently seeking an election for are Northwest workers who are currently represented by the machinists. The Delta workers are not unionized.

The machinists union also represents Northwest stock clerks, passenger service and office and clerical workers. Tiberi said the union expected to seek an election for those employees at a future date.

Last month, the Association of Flight Attendants-CWA union took a similar action in a bid to resolve representation for more than 20,000 employees at Delta. Northwest has 7,350 flight attendants currently represented by the AFA, while Delta's 13,300 are not unionized, the AFA-CWA said.

Delta shares were off 7 cents, or 1 percent, at $7.13 in morning trading. (Reporting by Karen Jacobs; Editing by Lisa Von Ahn)