More than 3,200 Boeing defense workers began a strike early Monday after rejecting a revised four-year contract offer that the company said included average wage increases of 40%, a $5,000 ratification bonus, and other benefits.

The walkout, led by the International Association of Machinists and Aerospace Workers' District 837, affects Boeing's fighter jet manufacturing operations across St. Louis and St. Charles, Missouri, and Mascoutah, Illinois. Workers at these facilities assemble and maintain the F-15 and F/A-18 fighters, the T-7 trainer jet, and the MQ-25 refueling drone.

Dan Gillian, vice president and general manager of Boeing's St. Louis defense facilities, said in a statement, "We're disappointed our employees in St. Louis rejected an offer that featured 40% average wage growth and resolved their primary issue on alternative work schedules."

Union leaders said the offer didn't meet expectations. "IAM District 837 members have spoken loud and clear, they deserve a contract that reflects their skill, dedication, and the critical role they play in our nation's defense," District 837 representative Tom Boelling said.

Boeing has activated a contingency plan to keep operations running with non-striking workers. The company emphasized it was prepared for the work stoppage, which follows last year's seven-week strike by District 751 commercial aircraft workers in Washington state. That strike ended in a 38% wage increase.

Boeing CEO Kelly Ortberg told analysts during the company's second-quarter earnings call, "I wouldn't worry too much about the implications of the strike. We'll manage our way through that."

Still, the strike comes at a sensitive time for Boeing. The aerospace giant has been working to stabilize its operations following recent crises, including a high-profile lawsuit over a midair panel blowout on an Alaska Airlines 737 earlier this year.