NEW YORK CITY, New York: Thousands of Boeing workers who build the company's fighter jets are preparing to go on strike, after voting down a modified labor agreement that union leaders say falls short of addressing key concerns around job security and benefits.

Roughly 3,200 workers across Boeing's facilities in St. Louis, St. Charles, Missouri, and Mascoutah, Illinois, plan to walk off the job starting at midnight Central Time on Monday, the International Association of Machinists and Aerospace Workers (IAM) announced Sunday.

"IAM District 837 members build the aircraft and defense systems that keep our country safe," said Sam Cicinelli, the union's Midwest territory general vice president. "They deserve nothing less than a contract that keeps their families secure and recognizes their unmatched expertise."

The vote marks the second time in recent weeks that members have rejected a Boeing proposal. The latest offer came after a previous deal, described by union leaders at the time as a "landmark agreement", was also turned down. That earlier version included a 20 percent wage increase over four years, as well as improvements to medical, pension, and overtime benefits.

After a one-week cooling-off period, members once again rejected Boeing's revised proposal, triggering plans for a strike.

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

The strike action adds to a turbulent period for Boeing, which has faced ongoing scrutiny over safety and financial performance following a string of high-profile incidents.

The company's reputation was first rocked by the fatal crashes of two 737 Max aircraft in 2018 and 2019, one in Indonesia, the other in Ethiopia, that killed a total of 346 people. More recently, in June, a Boeing Dreamliner operated by Air India crashed, resulting in at least 260 fatalities.

Despite the setbacks, Boeing reported improved financial results last week. The company posted a second-quarter loss of US$611 million, down from a $1.44 billion loss during the same period the previous year, while revenue increased, offering some signs of stabilization.

Still, the looming strike by defense sector workers threatens to disrupt a critical line of business: Boeing's military aircraft division, which includes production of fighter jets and defense systems.

With no immediate deal in sight, both sides appear to be bracing for a prolonged standoff.