The expansion of the strike against Amazon by the International Brotherhood of Teamsters this weekend has exacerbated concerns about shipping delays ahead of the Christmas and Hanukkah holidays.

Workers at the KSBD air hub in San Bernardino, California, were the latest to join other union members in New York City, Atlanta, Illinois, San Francisco, and parts of Southern California striking against Amazon. The thousands of workers walking off the job days before Christmas has raised the possibility of delays, with one striking official arguing the company is “jeopardizing the holidays.”