Members of a union representing workers who assemble fighter jets in two Boeing factories have rejected the US aircraft manufacturer’s proposed new contract, paving the way for possible strikes.

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

More than 3200 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 expired at the end of the day on Sunday (local time). The workers, who are 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,” Dan Gillian, vice-president of Boeing Air Dominance and general manager of the site in St Louis, Missouri, said.

Workers felt the offer didn't secure their future. Boeing is preparing for a strike. Photo / Getty Images

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% pay increase over four years and more vacation days.

But members working at sites in St Louis and Mascoutah, Illinois felt the offer 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 labour strike.

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

– Agence France-Presse