SAN FRANCISCO - Approximately 3,200 Boeing union workers in the US states of Missouri and Illinois went on strike on Monday after contract negotiations with the company broke down.

The strike move was earlier announced after members of the International Association of Machinists and Aerospace Workers (IAM) District 837 voted to reject a modified four-year labor agreement with Boeing.

This vote followed members' overwhelming rejection of Boeing's earlier proposal on July 27 as a labor agreement officially expired before midnight.

The workers were based at Boeing's facilities in St. Louis and St. Charles, Missouri, as well as Mascoutah, Illinois.

"We will be there on the picket lines, ensuring Boeing hears the collective power of working people," said IAM International President Brian Bryant in the statement on Sunday.

"They deserve nothing less than a contract that keeps their families secure and recognizes their unmatched expertise," IAM Midwest Territory General Vice President Sam Cicinelli said.

"Solidarity is our strength. This vote shows that when workers stand together, they can push back against corporate greed and fight for a better future for themselves and their families," said IAM Resident General Vice President Jody Bennett.

The IAM union is one of North America's largest and most diverse industrial trade unions, representing approximately 600,000 active and retired members in the aerospace, defense, airlines, railroad, transit, healthcare, automotive, and other industries across the United States and Canada.

Boeing expressed disappointment over the vote. Dan Gillian, Boeing Air Dominance vice president and general manager, and senior St. Louis site executive, said in a statement that the company was "disappointed our employees rejected an offer that featured 40 percent average wage growth and resolved their primary issue on alternative work schedules."

Boeing was "prepared for a strike and had fully implemented our contingency plan to ensure our non-striking workforce can continue supporting our customers," he added.