"IAM Union members delivered a clear message: the proposal from Boeing Defense fell short of addressing the priorities and sacrifices of the skilled... workforce," the International Association of Machinists and Aerospace Workers said in a statement.

More than 3,200 members of the union employed at Boeing facilities in the midwestern states of Missouri and Illinois rejected the company's contract offer.

According to the union, their contract expires at the end of the day Sunday. The workers part of IAM branch 837 will go on strike if no deal is reached with the aerospace giant in the next seven days.

The move could deal a serious blow to the company, after a seven-week long walkout by Seattle-based workers last year crippled two of Boeing's major assembly plants.

"We're disappointed our employees voted down the richest contract offer we've ever presented to IAM 837 which addressed all their stated priorities," said Dan Gillian, vice-president of Boeing Air Dominance and general manager of the site in St. Louis, Missouri.

In a statement sent to AFP, Gillian said no talks were scheduled with the union and the company was "preparing for a strike."

Boeing's proposal included a 20 percent pay increase over four years and more vacation days.

But members working at sites in St. Louis and Mascoutah, Illinois felt it did not meet their needs and did not guarantee a "secure future," according to the union.

In March, President Donald Trump announced that Boeing had been awarded a major contract for the US Air Force's next-generation F-47 fighter planes, the replacement for the F-22 which has been in operation for some two decades.

That announcement came as a boon for Boeing after a difficult year in which it struggled with safety problems and a lengthy labor strike.

IAM is one of North America's largest unions, representing members in aerospace, defense, transport manufacturing and other industries.

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