By David Shepardson

(Reuters) - Talks between the United Auto Workers and the Detroit Three automakers resumed on Saturday, a day after the union began its first simultaneous strikes at three U.S. auto plants.

The four-year labor deal between the union and General Motors (NYSE:), Ford Motor (NYSE:) and Chrysler-parent Stellantis (NYSE:) expired at 11:59 p.m. EDT on Thursday. Stellantis said on Saturday it has hiked its offer, proposing cumulative raises of nearly 21% over a four-and-a-half-year contract term, including an immediate 10% hike.

GM and Ford are also offering wage hikes of 20% over the same period.

The strikes, which mark the most ambitious U.S. industrial labor action in decades, have halted production at three plants that produce the Ford Bronco, Jeep Wrangler and Chevrolet Colorado, along with other popular models.

Automakers say they need cost-competitive contracts because of the need to spend billions of dollars to make the transition to electric vehicles (EV), while workers note U.S. automakers have enjoyed robust profits over the last decade and had hiked CEO salaries by 40% on average since 2019.

On Friday, Ford said it was indefinitely laying off 600 workers at a Michigan plant because of the impact of the strike at the plant, which makes the Bronco SUV, and GM told some 2,000 workers at a Kansas car plant that their factory likely would be shut down next week for lack of parts, stemming from a nearby Missouri plant being struck.

UAW President Shawn Fain called the reports of planned layoffs of non-striking workers an attempt by the automakers to "squeeze" union members into accepting a weaker settlement.

"Their plan won't work," Fain said in a statement. "We'll organize one day longer than they can and go the distance to win economic and social justice at the Big Three."

The union is demanding higher wages, shorter work weeks, restoration of defined benefit pensions and stronger job security as automakers make the EV shift.

Stellantis said it is offering more than $1 billion in retirement security improvements and other increases in benefits.