Boeing withdrew a contract offer for 33,000 machinists who have been on strike since mid-September, and said further negotiations "do not make sense at this point."

The machinists walked off the job on Sept. 13 after overwhelmingly rejecting a tentative labor deal, halting production of most of Boeing's aircraft, which are made in the Puget Sound area. Boeing later sweetened the offer, increasing pay raises, a ratification bonus and other improvements, which the union turned down, arguing that it was not negotiated.

Talks again broke down this week, meaning the strike will continue. The stoppage will cost Boeing more than $1 billion per month, S&P Global Ratings said Tuesday as it issued a negative outlook for the aerospace giant's credit ratings.

Stephanie Pope, CEO of Boeing's commercial aircraft unit, said the company improved contract pay during talks this week but said the union didn't consider the proposals.

"Instead, the union made non-negotiable demands far in excess of what can be accepted if we are to remain competitive as a business," Pope said in a staff note.

The union, the International Association of Machinists and Aerospace Workers, said Tuesday that Boeing refused to improve wages, retirement plans and vacation or sick leave.