Boeing Defense plans to hire new workers to replace striking members of the International Association of Machinists and Aerospace Workers (IAM) who assemble munitions, fighters and other military aircraft in the St. Louis area, the company said in a message to employees on Thursday.

"Today, we’re starting the process to hire permanent replacement workers for manufacturing roles," Boeing Defense Vice President Dan Gillian said in a message to employees on Thursday. "This will ensure we’re properly staffed to keep supporting our customers."

Union leaders urged Boeing to resume negotiations.

IAM International President Brian Bryant said in a statement that "Boeing is doubling down on its mismanagement by saying it plans to hire replacement workers to build military aircraft and equipment, instead of negotiating with their dedicated, generational and skilled workforce."

The company has used non-union employees to keep production going since the strike began on August 4, after the 3,200 members of IAM District 837 voted 67% to reject the company's latest four-year contract offer. Production has slowed on some programs, Gillian said during a press briefing on Wednesday.

Boeing is open to "minor adjustments" to the offer rejected by IAM members, not substantial ones, he said during the briefing.

Job openings are being posted on Thursday, followed by a job fair on September 16, he said in Thursday's message, which was shared with Reuters.

"Once these new manufacturing employees are hired, they will undergo the same training and certifications that we require of all our teammates," he said.

That would include getting security clearances for some jobs, such as working on final assembly for the F/A-18 fighter.

Getting a clearance usually takes about six months, IAM District 837 President Tom Boelling told Reuters.

Boeing declined to comment on whether it plans to hire for jobs requiring a security clearance or how long it takes for employees to obtain one.

Federal labour laws allow a company to permanently replace workers on strike over contract negotiations, said Sharon Block, a former member of the National Labour Relations Board and a labour law professor at Harvard Law School.

Boeing Defense spokeswoman Didi VanNierop said that if a union member's job has been filled, then "they will be added to a recall list until a position for which they are qualified becomes available."

However, replacement workers can be laid off as part of a strike-ending deal between union members and an employer, Block said.

Boeing has invested billions of dollars to expand manufacturing facilities and engineering capabilities in the St. Louis area for the new U.S. Air Force fighter jet, the F-47A. It won the contract this year.

The company is also competing for the U.S. Navy's new F/A-XX fighter. The striking machinists are "the core that will help us do all of that," Gillian said in Wednesday's briefing.

The offer rejected by St. Louis-area workers included a 20% general wage increase, faster wage progression, more vacation time and sick leave, and a $5,000 ratification bonus. Boeing withdrew the bonus after the offer was rejected. The average compensation increase for union members would be 40% over the contract's duration, according to Boeing.

A seven-week strike by IAM District 751 members in Washington and Oregon ended with a contract that included a 38% wage increase and a $12,000 signing bonus. Boeing then gave a $12,000 bonus to its non-union workers in South Carolina.

"It’s a slap in the face to not offer the same to our hard-working, dedicated IAM District 837 membership in the St. Louis area," Bryant said.