Boeing workers in the Seattle region decisively rejected the US aerospace giant's latest contract offer on Wednesday, extending their nearly six-week strike.

Almost two-thirds – 64 percent – of the members of the International Association of Machinists and Aerospace Workers District 751 rejected the contract, the union said on social media platform X.

The latest Boeing offer had included a 35 percent wage hike, but did not reinstate a pension plan sought by many employees.

Some 33,000 hourly workers with the IAM have been on the picket line since September 13, when workers overwhelmingly rejected a Boeing proposal for a new four-year contract to replace the expiring pact.

The strike has halted activity at two Seattle-area factories that assemble the 737 MAX and 777.

Workers had sought a 40 percent wage increase to make up for years of tepid salary growth that have not kept pace with inflation and that employees complain leave them unable to afford living in one of the most costly regions of the United States.

"After 10 years of sacrifices, we still have ground to make up, and we're hopeful to do so by resuming negotiations promptly," Jon Holden, president of the Seattle union, said in a statement.

"This is workplace democracy – and also clear evidence that there are consequences when a company mistreats its workers year after year," Holden said.

"Ten years of holding workers back unfortunately cannot be undone quickly or easily, but we will continue to negotiate in good faith until we have made gains that workers feel adequately make up for what the company took from them in the past," he added.

A Boeing spokeswoman said Wednesday night the company had no comment on the vote.

The extension of the strike adds to the troubles facing Boeing and its new CEO Kelly Ortberg, who earlier Wednesday expressed measured optimism the latest contract would be ratified. (AFP)