Negotiators for the United Auto Workers (UAW) and Ford Motor had "reasonably productive discussions" toward a new contract, the union said on Saturday, while officials at Chrysler parent Stellantis said a proposal to resume work at an idled Illinois factory has fallen through.

About 12,700 UAW workers remained on strike for a second day as part of a coordinated labor action targeting three U.S. assembly plants, one at each of the Detroit Three automakers.

Union negotiators and representatives of General Motors (GM), Ford and Stellantis resumed talks on Saturday, a day after the UAW initiated the most ambitious U.S. industrial labor action in decades.

However, there was no sign of a breakthrough as of Saturday afternoon.

The first ever strike of all three at once in the union's 88-year history came after it failed to clinch a deal on a new contract by a deadline on Thursday. 

The union had reportedly been negotiating with all three automakers simultaneously, in a break from previous rounds of contract talks, for significantly higher pay and new benefits.

Particularly, the UAW called for protections for workers with traditional auto jobs as companies increasingly invest in EV production.

Stellantis said on Saturday it had hiked its offer, proposing raises of 20 percent over a four-and-a-half-year contract term, including an immediate 10 percent hike. That matches proposals from GM and Ford. Stellantis said talks would resume on Monday.

The automakers said the proposals work out to a cumulative 21 percent hike over the period, but they are still significantly below the 40 percent wage hike the UAW is demanding through 2027. The union's wage demand includes a 20 percent immediate increase.

The automakers have also said the UAW demands could hike the current mid-$60-per-hour labor cost to more than $150 an hour. GM said on Thursday the UAW wage and benefits proposals would cost it $100 billion, while Ford's Farley said a 40 percent UAW wage hike would "put us out of business."

In response to the UAW strike, U.S. President Joe Biden urged the Big Three automakers to "go further" in their offers to the UAW. In a televised speech, Biden noted the massive profits made by automakers in recent years that "have not been shared fairly" with workers.

"The companies have made some significant offers. But I believe it should go further to ensure record corporate profits mean record contracts for the UAW," he said.

Analysts warned that the strike, though limited for the time being, may develop into a prolonged one and a menace to the U.S. economy, as it could cut production by thousands, push up vehicle prices and aggravate supply chain disruptions.

(With input from agencies)