The U.A.W. and G.M. are at odds over pay for temporary and recently hired workers, health care costs, the automaker’s intention to shut down four plants in the United States, and decisions that have shifted a larger portion of its production to Mexico in recent years.

[Read more: Here are key points in the dispute between G.M. and its unionized workers.]

In a letter to union members Thursday evening, Terry Dittes, a U.A.W. vice president, said the two sides were prepared to continue bargaining, through the weekend and beyond, in the absence of a tentative agreement.

“I can report to you that as of today, some progress has been made, but there are still many of our membership’s issues that remain unresolved,” Mr. Dittes said.

As the discussions continue, the walkout is beginning to have an impact on G.M.’s Canadian operations. The company has stopped production at its plant in Oshawa, Ontario, idling about 2,000 members of the Canadian union, Unifor. Elsewhere in the province, G.M. is evaluating whether to idle an engine plant in St. Catharines, which employs 700 people, and at a plant in Ingersoll that makes sport utility vehicles, a G.M. spokesman said.