increased its wage proposal to tens of thousands of striking workers on Monday, offering a 30% general wage increase over four years in what it called its "best and final" offer as a work stoppage that began on Sept. 13 drags on.

The U.S. planemaker also offered to reinstate a performance bonus, improve retirement benefits and double a ratification bonus to $6,000 if the workers accept the offer by Friday, according to a letter sent to union officials by the company.

It is unclear if the new proposal will satisfy the more than 32,000 Boeing workers in Portland and the Seattle area who walked out in Boeing's first strike since 2008. The workers, who have sought 40% higher pay as well as the restoration of the performance bonus, rejected a previous offer by the company.

Brian Bryan, the president of the International Association of Machinists and Aerospace Workers, said the union is reviewing the company's latest proposal. The union represents the workers who build Boeing's best-selling 737 MAX and other jets.

Boeing is under intensifying pressure to end the strike, which could cost it several billion dollars, fraying the company's already-strained finances and threatening a downgrade of its credit rating. The two sides ceased bargaining last week.

A company spokesperson said on Monday, "After listening to our employees and their concerns, Boeing today presented our best and final offer" to union leaders. Boeing's commercial planes chief Stephanie Pope had told workers before the strike that the company had held nothing back and that its offer at that time was the best deal they would get.

"Employees knew Boeing executives could do better, and this shows the workers were right all along," Bryan said in a statement.

The strike is the latest event in a tumultuous year for the company that began with a January incident in which a door panel detached from a new 737 MAX jet mid-air.

An earlier tentative deal between Boeing and the union that offered a 25% raise over four years and a commitment that a new plane would be manufactured in the Seattle area if it were launched during the four-year agreement was voted down by more than 90% of workers this month.

Boeing has frozen hiring and started furloughs for thousands of U.S. employees to reduce costs amid the strike. Boeing has planned for workers to take one week of furlough every four weeks on a rolling basis for the duration of the strike.

The extensive furloughs show that new CEO Kelly Ortberg is preparing Boeing to weather a prolonged strike that may not be easily resolved given the anger among rank-and-file workers.

North American unions have capitalized on tight labor markets to win hefty contracts at the bargaining table, with mainline pilots, auto workers and others scoring big raises in 2023.

The International Association of Machinists and Aerospace Workers said that 5,000 of its members in Wichita, Kansas went on strike against Cessna business jet maker Textron (TXT.N), opens new tab starting on Monday.