United Airlines fleet service, passenger service and storekeeper employees rejected tentative labor agreements reached with airline management in February.

The inability of United to secure a unified labor contract with more than 28,000 workers from several work groups representing legacy United and Continental Airlines employees marked another setback for the carrier, which has struggled with labor and technology integration issues since the fall 2010 merger of United and Continental. Integration problems contributed to a large net loss incurred by United in 2012.

The International Association of Machinists (IAM) said in a statement the workers “emphatically rejected the tentative agreements reached with United Airlines” in voting conducted across the US. The union said it is notifying the US National Mediation Board “of our request to enter into federally mediated negotiations … Negotiations will resume based on the directives of the National Mediation Board.”

United said in a statement, “We believe these tentative agreements are in the best interests of [the employees] and the company. We will work with the IAM and National Mediation Board to determine our next steps.”

Of the various United work groups, only its pilots have ratified a joint labor agreement covering both legacy United and Continental employees.