United Auto Workers (UAW) union members at General Motors (GM) have walked out on strike after contract talks failed to reach an agreement.
The UAW says it will not suspend the strike without an agreement
It is the first time that UAW has held a nationwide strike during contract negotiations since 1976.
Workers began forming pickets outside GM plants in the US as the deadline for a deal passed.
UAW president Ron Gettelfinger said he expects to return to negotiations with GM later on Monday.
The carmaker wants staff to give up some of their expensive benefits such as pensions and health insurance.
The talks apparently became bogged down on Sunday because the union wanted guarantees that new vehicles and models will be built in the US.
Mr Gettelfinger confirmed that job security had indeed been one of the major remaining issues.
But he denied that the proposed voluntary employee benefit association or Veba had been a stumbling block.
The Veba would take responsibility for the healthcare benefits of 460,000 retirees away from GM.
Mr Gettelfinger said UAW had been "more than eager" to discuss it.
In a statement, GM said it was disappointed about the strike.
"The bargaining involves complex, difficult issues that affect the job security of our US work force and the long-term viability of the company," it said.
"We will continue focusing our efforts on reaching an agreement as soon as possible," it added.
GM employs 73,000 UAW members whose contracts expired on Friday, 14 September.
Since then they have been working without a formal contract as talks continued.
The strike does not affect GM workers outside North America.
The UAW wants the talks with GM to come up with a contract that can then be followed by Chrysler and Ford.
"I don't think there's any big, hidden pile of money that management is keeping from the unions, so it's a little bit surprising that they would have gone to this extent," said Peter Jankovskis from Oakbrook Investments in Illinois.