Boeing and its largest union will restart contract talks o Wednesday in the presence of federal mediators, after failing to agree on key issues such as wages and pensions, the International Association of Machinists and Aerospace Workers said.

The union, whose members went on strike last Friday, has been pushing for a 40% raise over four years in its first full contract negotiations with Boeing in 16 years, well above the planemaker's offer of 25%, which was resoundingly rejected.

A prolonged strike could cost Boeing several billion dollars, further straining the planemaker's finances and threatening a downgrade of its credit rating, analysts said.

"After a full day of mediation, we are frustrated, the company was not prepared and was unwilling to address the issues you've made clear are essential for ending this strike: Wages and Pension," the union representing more than 30,000 Boeing factory workers said on X following Tuesday's meeting.

"The company doesn't seem to be taking mediation seriously. With a 96% strike vote, we thought Boeing would finally understand that IAM 751 Machinists are demanding more. We are fighting for what is right and just - for what we have earned over the past 16 years," it added.

The strike, which entered its sixth day on Wednesday, is Boeing's first since 2008 and is the latest event in a tumultuous year for the planemaker that began with a January incident when a door panel detached from a new 737 MAX jet in mid-air.

Boeing and the U.S. Federal Mediation & Conciliation Service did not immediately respond to emails seeking comment outside normal business hours.

The strike has halted production of Boeing's best-selling 737 MAX jets, along with its 777 and 767 widebody aircraft, delaying deliveries to airlines.

Boeing said on Monday it was freezing hiring and weighing temporary furloughs to cut costs as its balance sheet is already burdened with $60 billion in debt and a prolonged strike could damage it further.

The company has also stopped placing most orders for parts for all Boeing jet programs except the 787 Dreamliner, in a move that will hurt its suppliers.

Shares are down about 40% this year.