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The union representing thousands of striking Boeing workers has hit out at what the aircraft manufacturing giant called its "best and final" pay offer, which proposed a 30% rise over four years.

The new offer also included the reinstatement of a performance bonus and improved retirement benefits.

However, the International Association of Machinists and Aerospace Workers (IAM) said the offer was not negotiated with the union and that "it was thrown at us without any discussion" - a claim Boeing denies.

More than 30,000 Boeing workers went on strike earlier this month after rejecting a 25% pay rise offer.

"After listening to our employees and their concerns, Boeing today presented our best and final offer," Boeing said in a letter.

The proposal doubles the value of a one-off bonus for signing a new pay deal to $6,000 (£4,497).

The company said the offer is dependent on it being ratified by union members by midnight pacific time on Friday 27 September (7am GMT on Saturday 28 September).

But IAM said Boeing sent the new offer directly to union members and the media without telling the union's representatives.

"This tactic is a blatant show of disrespect to you - our members - and the bargaining process," .

The union also said it would not hold a vote of its membership ahead of Boeing's deadline.

In response, Boeing told the ´óÏó´«Ã½: "We have bargained in good faith with the IAM since formal negotiations began in March."

"We first presented the offer to the union and then transparently shared the details with our employees," it added.

Boeing workers went on strike from 13 September after rejecting a new contract deal, which included a 25% pay rise over four years.

The union had initially aimed for a number of improvements to workers' packages, including a 40% pay rise.

Almost 95% of the union members - who produce planes including the 737 Max and 777 - voted to reject Boeing's initial offer.

Of those who voted, 96% backed strike action until a new agreement could be reached.

The strike threatens to cost Boeing billions of dollars, deepening the crisis at a company already facing significant challenges.

Its impacts are already being felt across the industry and wider US economy too, as Boeing has halted shipments of most parts and taken other steps to save money.

The company has already suspended the jobs of tens of thousands of staff.

It has also said that US-based executives, managers and staff would be asked to take one week of furlough every four weeks for as long as the walkout lasts.

Government officials are now helping to mediate talks between the two sides.