More than 3,200 union members who assemble Boeing's fighter jets in the St. Louis area and Illinois went on strike on Monday after rejecting a second contract offer the previous day. Boeing Defense said it was ready for the work stoppage and it will implement a contingency plan that uses non-labor workers.

E mployees at Boeing’s St. Louis defense factories rejected a contract offered by the aerospace giant Sunday afternoon and around 3,200 workers began striking starting at midnight—the first walk out the company has faced at its St. Louis defense hub since 1996.

Roughly 3,200 workers are striking at sites in Missouri and Illinois. Members of the labor union representing the workers rejected a modified contract offered by Boeing last week.