"We respectfully ask you to join us in supporting this vitally important, highly skilled workforce," the machinist union said in letters to the Missouri congressional delegation, describing Boeing's latest offer as "substandard."

Some 3,200 members of the International Association of Machinists & Aerospace Workers District 837 have been on strike since August 4 after rejecting a contract proposal. The two sides have not held talks since the walkout, representatives of both sides said.

The strike affects Boeing facilities in St. Louis and St. Charles, Missouri and Mascoutah, Illinois.

"We ask you to urge the Boeing Company to promptly return to the bargaining table with new ideas and proposals in order to seek a fair and equitable settlement to this disagreement," said a letter signed by IAM International President Brian Bryant.

Products produced at the affected sites include the F-15 and F-18 combat aircraft, the T-7 Red Hawk Advanced Pilot Training System and the MQ-25 unmanned aircraft.

Prioirties for the union include fair compensation, with wages that meet the cost of living and "a contract that respects seniority and experience," the IAM said.

Boeing said its offer includes 40 percent average wage increases, as well as more vacation and sick leave.

"We're surprised to hear the IAM International call our offer 'substandard' after hailing it as a 'landmark' agreement which they endorsed just three weeks ago," said Boeing Air Dominance vice president Dan Gillian.

"We remain ready and willing to listen to any constructive proposals from the union."

The St. Louis local's stoppage follows a bruising Boeing strike last fall in the Pacific Northwest region of some 33,000 workers that halted production at factories that assemble Boeing commercial planes.

Boeing executives have characterized the St. Louis strike as manageable, noting that they had prepared for a stoppage before the workers joined the pickets.