The roughly three dozen affected specialist and administrative workers at UPS's Centennial hub in Louisville, Kentucky, organized with Teamsters Local 89 this autumn, the union said.
International Brotherhood of Teamsters General President Sean O'Brien in a statement said that UPS laid off those workers despite the ruling of an independent arbitrator and falsely claimed that their work should be performed by management.
"If UPS doesn't get its act together, they'll be on strike next. Our union will not hesitate to act, and we will not back down," said O'Brien, who represented some 340,000 UPS workers in a contract deal reached earlier this year.
UPS did not immediately respond to a request for comment.
(Reporting by Lisa Baertlein in Los Angeles; Editing by Bill Berkrot)