Mr. Dittes said the union responded Tuesday with a counterproposal, but did not offer details.

Until recently, the two sides had been meeting in individual committees to focus on particular topics such as temporary workers and health care. Overall proposals are presented at what is known as the main table, where the lead negotiators face off in a single room.

“We continue to negotiate and exchange proposals, and remain committed to reaching an agreement that builds a stronger future for our employees and our company,” G.M. said in a statement Tuesday.

[A look back: Landmark auto strikes since the 1930s.]

Both sides are feeling the impact of the walkout, the longest against G.M. in half a century. On Tuesday, G.M. said it had halted production of pickup trucks, engines and transmissions at plants in Silao, Mexico, because of a shortage of parts that are produced in United States plants. The company’s other factories in Mexico are still operating, a G.M. spokesman said.

Part shortages previously forced G.M. to stop production at a vehicle assembly plant in Oshawa, Ontario, and an engine plant in St. Catharines, Ontario.

Analysts say the strike is costing General Motors tens of millions of dollars per day.

The 49,000 union workers at G.M. are entering their third week without regular paychecks. The U.A.W. is providing each worker $250 a week in strike relief.