Boeing in a statement said it was "disappointed" but was also prepared.

A F/A-18E/F Super Hornets of Strike Fighter Attack Squadron 211 (VFA-211) is lined up for take off on the flight deck of the USS Theodore Roosevelt (CVN-71) aircraft carrier in the Gulf June 18, 2015.

A F/A-18E/F Super Hornets of Strike Fighter Attack Squadron 211 (VFA-211) is lined up for take off on the flight deck of the USS Theodore Roosevelt (CVN-71) aircraft carrier in the Gulf June 18, 2015.

A F/A-18E/F Super Hornets of Strike Fighter Attack Squadron 211 (VFA-211) is lined up for take off on the flight deck of the USS Theodore Roosevelt (CVN-71) aircraft carrier in the Gulf June 18, 2015.

A F/A-18E/F Super Hornets of Strike Fighter Attack Squadron 211 (VFA-211) is lined up for take off on the flight deck of the USS Theodore Roosevelt (CVN-71) aircraft carrier in the Gulf June 18, 2015.

About 3,200 union members at Boeing facilities in Missouri and Illinois went on strike at midnight on Monday after rejecting an contract offer from the company, the union said.

Local members of the International Association of Machinists and Aerospace Workers, who build and maintain fighter jets, including the F-15 and F/A-18 models, voted on Sunday to reject Boeing's latest contract offer.

"IAM District 837 members build the aircraft and defense systems that keep our country safe," IAM Midwest Territory General Vice President Sam Cicinelli said in a statement.

U.S. President Donald Trump holds a round table on the economy during his visit to Boeing in St. Louis, Missouri, U.S. March 14, 2018. Kevin Lamarque/Reuters

Cicinelli added, "They deserve nothing less than a contract that keeps their families secure and recognizes their unmatched expertise."

The union members work at Boeing facilities in St. Louis and St. Charles, Missouri, along with Mascoutah, Illinois, according to the union.

They had voted on July 27 to reject an earlier 4-year contract proposal put forward by the company, the union said.

A F/A-18E/F Super Hornets of Strike Fighter Attack Squadron 211 (VFA-211) is lined up for take off on the flight deck of the USS Theodore Roosevelt (CVN-71) aircraft carrier in the Gulf June 18, 2015. Hamad I Mohammed/Reuters

"We're disappointed our employees rejected an offer that featured 40% average wage growth and resolved their primary issue on alternative work schedules," Dan Gillian, Boeing Air Dominance vice president and general manager and senior St. Louis site executive, said in a statement on Sunday. "We are prepared for a strike and have fully implemented our contingency plan to ensure our non-striking workforce can continue supporting our customers."

Boeing officials noted that their contingency planning ensures there are no security issues related to using a non-union workforce, though impacts to operations will depend on the length of the strike.

Editor’s note: This article previously misstated one of the two states where Boeing workers have gone on strike.