The company, which is bidding to revive its fortunes under a new chief executive, has more than 32,000 staff refusing to work.

They walked out of factories in the Seattle area and in Oregon, which make aircraft including its 737 MAX models, on 13 September in protest at new contracts.

The International Association of Machinists and Aerospace Workers (IAM) is demanding the workers get a 40% pay rise.

It is also seeking the restoration of a defined benefit pension plan.

Boeing made this week, what it said was, a "best and final offer" of a 30% pay rise over four years and a performance-related bonus.

The union has refused to put the offer to a vote, declaring it falls short of what its members find acceptable.

Two previous days of negotiations ended without agreement.

Boeing placed tens of thousands of other workers on furlough as the strike began.

Production of its best-selling planes, the 737 MAX models, had already been hurt by curbs imposed by regulators in the wake of the mid-air Alaska Airlines scare in January that saw a panel blow out, forcing the 737 MAX 9 plane into an emergency landing.

The incident prompted renewed safety scrutiny of Boeing after the scandal that followed the fatal crashes of MAX 8 planes in 2018 and 2019, blamed on faulty flight control software.

The production limits imposed on Boeing this year have further harmed deliveries to customers globally, including Ryanair.

Europe's largest carrier by passengers, which has blamed Boeing's delays for cuts to its expansion plans this year, made clear it feared extended delays to deliveries of new planes this week when asked about the impact of the strikes.

"The union is ready for this opportunity to bring forward the issues that members have identified as critical to reaching an agreement," IAM said in a statement.

"We know that the only way to resolve this strike is through negotiations."

Boeing, which has seen its shares fall 40% in the year to date, did not comment ahead of the talks.

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Susannah Streeter, head of money and markets at Hargreaves Lansdown, said of the dispute this week: "Boeing remains deep in a wage row, which is spelling fresh production trouble ahead.

"Shareholders who hoped that the arrival of a new CEO Kelly Ortberg in August would help the company make a fast exit from its difficulties are sorely disappointed. The deep-seated nature of the challenges the company is facing has come even starker, given the rejection of 30% pay offer to staff.

"Relations between management, union leadership and staff appear to have broken down further, given complaints by workers that they weren't given time to vote on the latest proposal."