More than 3,200 union members who assemble Boeing’s fighter jets in the St. Louis, Mo., 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.

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.