Separately, the UAW confirmed it presented a new contract offer to GM on Monday. GM said it received the counterproposal "but significant gaps remain." The UAW also held a new round of bargaining with Chrysler-parent Stellantis.

Ford on Monday said it was furloughing a total of 330 workers at its Chicago Stamping and Lima, Ohio Engine plants, while GM layoffs included 130 at its Parma, Ohio Metal Center and 34 at its Marion, Indiana Metal Center.

The UAW on Friday struck GM's Lansing Delta Township assembly plant that makes the Buick Enclave and Chevrolet Traverse SUVs after previously striking at GM's Missouri assembly plant and 18 parts distribution centers.

GM last month said it had idled its Fairfax, Kansas, plant because of a parts shortage stemming from the strike, resulting in 2,000 indefinite layoffs. Ford said last month it temporarily laid off about 600 workers at a Michigan auto plant because of the strike. Stellantis has furloughed nearly 370 workers in Ohio and Indiana.

UAW President Shawn Fain on Friday expanded the first-ever simultaneous strike against the Detroit Three to a GM Lansing, Michigan plant and a Ford Chicago assembly plant, but Stellantis was spared after last-minute concessions.

The UAW said it reached a new labor agreement with Volvo Group's Mack Trucks covering 4,000 workers just before a Sunday night deadline.

The tentative agreement that includes significantly increased wages must still be ratified, the union said.

JPMorgan in a research note on Monday estimated the strike has cost GM $191 million and Ford $145 million, but said there was some cause for optimism about a deal, citing reports that the two sides were "close on pay and benefits."

The GM and Ford CEOs on Friday blasted the UAW, hours after the union escalated the strike. The UAW responded on social media that neither had attended bargaining talks.

Anderson Economic Group estimated total losses from the first two weeks of the strike to be $3.9 billion, including $325 million in wages, $1.12 billion in losses for the Detroit Three, $1.29 billion for suppliers and $1.2 billion for dealer and customer losses.