(Reuters) -Union members who assemble Boeing's ( BA ) fighter jets in the St. Louis area rejected the U.S. planemaker's latest contract offer on Sunday and will strike at midnight on Monday, the International Association of Machinists and Aerospace Workers union (IAM) said.

"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.

Boeing ( BA ) said it was ready for the action. "We are prepared for a strike and have fully implemented our contingency plan," Boeing Air Dominance vice president and general manager Dan Gillian said in an emailed statement to Reuters.

"We're disappointed our employees in St. Louis rejected an offer that featured 40% average wage growth," he added.

Boeing's ( BA ) original proposal included a 20% general wage increase over four years and a $5,000 ratification bonus, as well as more vacation time and sick leave. The union had rejected the offer, saying it was insufficient.

Last week, Boeing ( BA ) 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 ( BA ) had proposed modifying in the last contract offer.

The workers assemble Boeing's ( BA ) fighter jets and the MQ-25, an aerial refueling drone being developed for the U.S. Navy.

Boeing's ( BA ) defense division is expanding manufacturing facilities in the St. Louis area for the new U.S. Air Force fighter, the F-47A, after it won the contract this year.

The upcoming strike by the union, representing 3,200 employees, would be much smaller than the one Boeing ( BA ) was hit with last fall, when 33,000 machinists at Boeing's ( BA ) commercial plane division walked out for nearly two months. That strike ended with approval of a four-year contract that included a 38% wage increase.