DETROIT, SEPT. 15 -- The pace of negotiations between General Motors Corp. and the United Auto Workers appeared to slow today as both sides focused on the union's demand for job security in an industry facing heavy foreign competition.

Negotiators returned to the bargaining table this morning hours after the union agreed to continue working past a midnight contract expiration. It was only the second time the union has agreed to a contract extension in auto negotiations.

The two sides are working against a Monday morning deadline, when union leaders have scheduled a meeting of the 300-member GM National Bargaining Council, which must approve a settlement before it can be submitted to the union rank and file for ratification.

A UAW official this afternoon said he was less optimistic today than Friday, when both sides indicated an agreement might be imminent. Major issues such as wages and pensions remain unresolved.

The key will be the job security program worked out for 280,000 active GM workers and 30,000 on indefinite layoff.

GM has insisted it needs to trim its union work force by 20,000 jobs in each of the next three years. The union wants to guarantee the jobs of the active workers and create some mechanism for bringing those on layoff back to work.

The union also wants to negotiate a formula that would guarantee minimum levels of employment at GM if the company's -- and the entire industry's -- economic fortunes turn around.

Any agreement reached with GM will be used by the union as a pattern for negotiations with Ford Motor Co. and Chrysler Corp. The UAW picked GM as a target for negotiating a contract pattern because, unlike Ford and Chrysler, it manufactures most of the parts it uses in assembling cars and trucks. The union wants to preserve that work and penalize the other two companies for contracting it out.

The three auto makers' biggest competition comes from the U.S.-based plants of Honda, Toyota and Nissan, which are nonunion, have lower labor costs and buy most of their parts from outside.

The current struggle focuses on the union's insistence on preserving jobs and the company's offer of income security for a large portion of the work force in exchange for GM's right to eliminate jobs.

The company initially offered to guarantee the jobs of UAW employees with 20 years or more of service, but the union insists the no-layoff guarantee be extended to all. There were indications today that GM might extend the guarantee to anyone with 15 years of service.

The average UAW worker at GM is 44 with 18 years service. UAW workers in the auto industry can retire with 30 years of service regardless of age.

In a related development today, 12,800 Canadian auto workers struck Ford of Canada.