Boeing said on Tuesday that it had withdrawn its pay offer to around 33,000 U.S. factory workers and no further negotiations were planned with their union representatives as a financially damaging strike nears its fourth week.

Boeing and the union held their latest round of negotiations with federal mediators on Monday and Tuesday, but talks collapsed and the sides were left locked in acrimonious stalemate showing no signs of being resolved anytime soon, a person briefed on the talks said.

"Unfortunately, the union did not seriously consider our proposals," Boeing Commercial Airplanes head Stephanie Pope said in a note to the employees, calling the union's demands "non-negotiable".

A strike sign hangs from a post near a Boeing sign as Boeing factory workers and supporters gather on a picket line during the third day of a strike near the entrance to a Boeing production facility in Renton, Washington, U.S. September 15, 2024. REUTERS/David Ryder

"Further negotiations do not make sense at this point and our offer has been withdrawn."

She noted Boeing had been taking steps to preserve cash.

Shares of the U.S. planemaker were down 1.5% in premarket trading on Wednesday. The stock has lost 5% of its value since the strike began on Sept. 13.

Reuters reported earlier on Tuesday that the planemaker is examining options to raise billions of dollars through a sale of stock and equity-like securities while the factories producing its best-selling 737 MAX and its 767 and 777 planes are shut.

The company, which is on the brink of losing its prized investment grade credit rating, has also introduced temporary furloughs for thousands of salaried employees.

The striking union of its West Coast factory workers is seeking a 40% pay rise over four years and the restoration of a defined-benefit pension that was taken away in the contract a decade ago. More than 90% of workers voted down an offer of a 25% pay rise over four years before going on strike.

Boeing made an improved offer last month that it described as its "best and final", which would give workers a 30% raise and restore a performance bonus, but the union said a survey of its members found that was not enough.

Pope, referring to the two days of negotiations this week, said: "Our team bargained in good faith and made new and improved proposals to try to reach a compromise, including increases in take-home pay and retirement."

In contrast, the International Association of Machinists and Aerospace Workers union said in a statement that Boeing was "hell-bent on standing on the non-negotiated offer" proposed last month.

"They refused to propose any wage increases, vacation/sick leave accrual, progression, ratification bonus, or the 401k Match/SCRC Contribution. They also would not reinstate the defined benefit pension," it said.