The United Auto Workers, UAW, union is expanding a strike at some of America’s biggest car firms, sending the row over pay and benefits into its third week.
Union boss Shawn Fain said 7000 more staff at Ford and General Motors factories are set to walk off the job.
He said the latest action did not target Stellantis, reflecting new momentum in those talks.
Roughly 18000 workers were already on strike in a dispute that has cast a cloud over the US economy.
US President Joe Biden and former President Donald Trump, who is running for re-election, both visited the Detroit area this week to address the issue, which comes as labour tensions simmer across the country.
The union opened talks seeking a roughly 40% rise in pay over four years and an end to practices that give newer hires lower pay and fewer benefits, among other demands.
The companies have maintained that the union’s requests would hurt their ability to invest in the long term.
They have countered with a roughly 20% pay increase and some other concessions.