United Auto Workers President Shawn Fain said the union's strike will expand if "serious progress" isn't made in the contract negotiations with automakers by Friday.
Fain said in a Monday evening update posted on social media that the deadline for greater progress in the union's talks with Ford, GM and Stellantis is Friday, Sept. 22, at noon.
"That will mark more than a week since our first members walked out. And that will mark more than a week of the 'big three' failing to make progress in negotiations toward reaching a deal that does right by our members," he said in his video message.
"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 added.
On Monday, the labor strike against the three largest motor vehicle manufacturers in the United States carried into a fourth day amid ongoing negotiations to reach a deal.
The UAW, which represents nearly 150,000 American autoworkers, launched a strike early Friday against General Motors, Ford and Stellantis -- often called the "big three." Almost 13,000 workers walked out of three auto plants in Michigan, Missouri and Ohio. The union is utilizing a "stand-up" strike method to target specific plants and add to the list if a deal isn't reached.
The UAW held talks with Ford on Saturday, GM on Sunday and planned to meet with Stellantis on Monday, a union source told ABC News. The conversations with Ford were "reasonably productive," the source said.
Sticking points in negotiations were wage increases and the length of the workweek. The union is demanding a 46% pay increase combined over the four-year duration of a new contract, as well as a 32-hour workweek at 40-hour pay. So far, all three of the Detroit-based companies have each put forward proposals that offered workers a 20% pay increase over the life of the agreement but preserved a 40-hour workweek.
After the unprecedented strike began on Friday, Ford laid off 600 workers who assemble cars at a plant in Michigan. Workers in the paint department at a nearby plant are out on strike, leaving the assembly workers without adequate parts since the parts require paint before they can be put together into cars, a company spokesperson told ABC News.
President Joe Biden said Friday he is deploying acting Labor Secretary Julie Su and White House senior adviser Gene Sperling to Detroit to offer their support for the parties in reaching an agreement.
Economists previously told ABC News that a strike could result in billions of dollars in losses, disruption to the supply chain and other financial consequences.
ABC News' Meredith Deliso, Jolie Lash and Max Zahn contributed to this report.