Employees at many Apple Inc. stores in the United States are working to unionize, the Washington Post reported on Friday, citing people familiar with the efforts.

The report said employee groups at at least two Apple retail stores are backed by major national unions and are preparing to file paperwork with the National Labor Relations Board (NLRB) in the near future.

At least six more locations are at less advanced stages in the unionization process, the report said.

Apple and the National Labor Relations Board did not immediately respond to Reuters requests for comment.