DETROIT – Bargainers for General Motors Corp. and the United Auto Workers took a break early today amid optimism that they are getting closer to reaching a critical contract agreement.

Negotiations came to an end just before 3 a.m. after a marathon 16-hour session yesterday and today, said GM spokesman Tom Wickham.

“GM and the UAW have agreed to take a break, and talks will resume later this morning. We aren’t going to comment or speculate on the nature or content of the discussions,” Mr. Wickham said.

He said that all employees were expected to report for work at their scheduled times today.

GM’s four-year contract with the UAW was to expire at midnight Friday, but the union extended it on an hour-by-hour basis.

Several local union leaders said Sunday that negotiators reported the talks were advancing. The leaders, some of whom asked not to be identified because of the confidential nature of the talks, said they were told by UAW leadership in Detroit that if no agreement was reached Sunday then the union would go on strike.

But as the talks continued into this morning, the strike threat seemed to wane.

A UAW local in Arlington, Texas, told its members to report to work as scheduled Monday but said it was committed to a strike if necessary. In a joint statement sent to union members and the media, Local 276 leaders told members they expected negotiators either to wrap up talks or declare an impasse at the end of yesterday’s negotiating session.

“We understand the issues are complex and the effects far-reaching,” local president Enrique Flores Jr. and shop chairman Dwayne Humphries said in the statement. “Solutions are certainly proving to be difficult.”

A message on a hotline at UAW Local 22 in Flint also told workers to report to work as scheduled Monday. The message said to ignore sign-up sheets for strike duty, a sign that a walkout wasn’t imminent.

UAW Local 735 President Chuck Rogers told his members yesterday that the UAW came within minutes of striking Friday night when President Ron

Mr. Gettelfinger walked out of a bargaining room after getting into a dispute with GM negotiators. But GM Chief Financial Officer Fritz Henderson intervened and brought Mr. Gettelfinger back to the bargaining table, and progress has been made since then, Mr. Rogers said.

Strike talk often is heard when negotiations get close to or pass the contract expiration deadline.

“I heard things are moving kind of in the right direction,” Dave Green, president of one of two locals at the Lordstown, Ohio, plant that makes small cars for

Chevrolet and Pontiac, said Sunday. “We let our folks know a strike is not out of the question,” he said, adding that he hoped for a resolution.

Bargainers also worked all day Saturday.

Messages were left for UAW spokesman Roger Kerson.

One of the local union leaders who asked not to be identified said the main outstanding issues were retiree health care expenses and whether GM would promise to build new vehicles at UAW-represented factories. GM wants the union to take over responsibility for retiree health care costs using a company-funded trust. The UAW was asking for job guarantees in exchange for taking on the costs.

GM, Ford Motor Co. and Chrysler LLC have a combined unfunded retiree health care obligation of more than $90 billion. GM’s unfunded obligation alone is $51 billion.

The local official said he was told Sunday that bargainers had moved closer on funding the health care trust, but were still apart on job security guarantees for factories.

The union named GM as the lead company and potential strike target Thursday, then extended contracts with Ford and Chrysler that can be ended by either party with three days’ notice. Once the union reaches an agreement with GM, it will pursue similar deals at Ford and Chrysler.

GM has about 73,000 UAW-represented hourly workers at its U.S. factories.