Boeing upped its wage proposal to 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 the strike drags on.
The U.S. planemaker is also offering to reinstate a performance bonus, improve retirement and double a ratification bonus to $6,000, if the workers accept by Friday.
Boeing has frozen hiring and started furloughs for thousands of U.S. employees to reduce costs. A prolonged strike could cost several billion dollars, fraying the planemaker's already strained finances and threatening a downgrade of its credit rating.
The strike, Boeing's first since 2008, is the latest event in a tumultuous year for the company that began with a January incident when a door panel detached from a new 737 MAX jet mid-air.
Factory workers rejected an earlier tentative deal between Boeing and the union, which 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.
A union spokesperson for the International Association of Machinists and Aerospace Workers was not immediately available for comment.