The auto workers' strike against Detroit's Big Three entered its fourth day with no signs of an early breakthrough and amid a threat by the United Auto Workers that the labor action could soon escalate. 

A spokesman for General Motors said that representatives of the company and the labor group were continuing to negotiate. But In a video statement late Monday, UAW President Shawn Fain said more factories could be targeted if "serious progress" toward an agreement isn't reached by Friday at noon.

"Autoworkers have waited long enough to make things right at the Big Three. We're not waiting around, and we're not messing around," he said.

So far the strike is limited to about 13,000 workers at three factories — one each at GM, Ford Motor and Stellantis. GM warned, however, that 2,000 UAW-represented workers at an assembly plant in Kansas City are "expected to be idled as soon as early this week" because of a shortage of supplies from a GM plant near St. Louis, where workers walked off the job Friday.

Workers at the Kansas City plant build the Chevrolet Malibu and Cadillac XT4.

Ford on Friday moved to temporarily lay off 600 non-striking workers at its assembly plant in Wayne, Michigan, only hours after other employees at the facility had walked off the job.

"This layoff is a consequence of the strike at Michigan Assembly Plant's final assembly and paint departments, because the components built by these 600 employees use materials that must be e-coated for protection," the company said in a statement Friday. "E-coating is completed in the paint department, which is on strike."

Treasury Secretary Janet Yellen said she is hoping for a quick resolution, and that it is too soon to gauge the impact of the strike.

"It's premature to be making forecasts about what it means for the economy. It would depend on how long the strike lasts and who would be affected by it," she said on CNBC.

Experts say the strike could drive up new and used car prices and cause a loss of $5.6 billion in wages and automaker earnings.

In a sign of the potential economic and political of a long strike, President Joe Biden is sending two top administration officials to Detroit this week to meet with both sides. Biden has sided with the UAW in brief public comments, saying that the automakers have not fairly shared their record profits with workers.

An administration official said Monday that acting Labor Secretary Julie Su and senior aide Gene Sperling will not serve as mediators — they won't be at the bargaining table — but are going to Detroit "to help support the negotiations in any way the parties feel is constructive." The official was not authorized to discuss private discussions and spoke anonymously.

The UAW's Fain on Sunday shot down an offer by Stellantis — which owns Chrysler, Dodge, Jeep and RAM, along with major foreign brands including Citroën, Peugeot and Maserati — to hike its worker' wages by 21% over four years. 

Ford and GM have also each offered a roughly 20% pay bump. The union is asking for a 36% hike over a four-year contract. 

The union also wants the Big Three automakers to eliminate their two-tier wage model, which results in many workers earning less than the average wage of $32 an hour; offer defined benefit pensions to all employees; limit the the use of temporary workers; offer a four-day workweek; and provide more job protections, including the right to strike over plant closings. 

"Our demands are just," Fain said on "Face the Nation." "We're asking for our fair share in this economy and the fruits of our labor." 

Rather than launching an all-out strike of its 146,000 members, the union opted to target three factories a plan that could make the union's $825 million strike fund last longer. Workers walked out of a GM plant in Wentzville, Missouri, a Ford plant near Detroit, and a Stellantis factory in Toledo, Ohio, that produces Jeeps.

A key feature of the UAW strategy is the threat of escalating the strike if the union is unhappy with the pace of bargaining. On Friday, Fain said more factories could be targeted: "It could be in a day, it could be in a week."

Strategically, targeting three factories "certainly created more uncertainty," Harry Katz, the Jack Sheinkman Professor of Collective Bargaining at Cornell University, told CBS News, adding that Fain is signaling that "he's a tough, militant guy that's not going to agree to concessions."

The UAW "will get a strong agreement — it's a question of how and when they reach a compromise," Katz predicted.

In:
• Detroit
• Janet Yellen
• United Auto Workers