More than 3,200 unionized workers who assemble fighter jets at Boeing’s facilities in the St Louis area have gone on strike after rejecting Boeing’s latest offer on August 3.

The workers assemble Boeing’s fighter jets and the MQ-25, an aerial refueling drone being developed for the U.S. navy. It’s the first strike that Boeing has faced at its St Louis defense hub since 1996.

The Guardian reports that Boeing Defense said it was ready for work stoppage August 4, and would implement a contingency plan that used non-labor workers. 

The strike could put added pressure on the already troubled planemaker, which generates about 30% of its revenue from its defense and space division, said The New York Times.

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“IAM District 837 members... deserve a contract that reflects their skill, dedication, and the critical role they play in our nation’s defense,” the union’s business representative, Tom Boelling, said on August 3.

Last week, Boeing sent a new contract offer to the union with some minor compensation changes that would benefit senior union members, according to the company. The offer also kept current overtime policies, which Boeing had proposed modifying in the last contract offer.

The offer was largely the same as the first offer that was overwhelmingly rejected one week earlier.

Boeing has said that if the contract offer had been approved, the average annual wage would have risen to $102,600 – up from $75,000.

“We’re disappointed our employees rejected an offer that featured 40 percent average wage growth and resolved their primary issue on alternative work schedules,” Dan Gillian, vice president of Boeing Air Dominance and general manager and senior executive at the St. Louis site, said in a statement emailed to media outlets.