DETROIT, SEPT. 10 -- General Motors Chairman Roger B. Smith today said it is virtually impossible to meet union demands for job security in the U.S. auto industry.

"I don't know of anybody who can do that," Smith said of United Auto Workers demands for fixed levels of employment at GM and Ford Motor Co.

The UAW is negotiating with the two auto makers to replace three-year labor agreements that expire at 11:59 p.m. Monday. Key issues at both companies involve job security for the union and improved productivity, at lower costs, for the manufacturers.

The union is attempting to keep its traditional practice of pattern bargaining: winning an agreement at one company, and then pressuring the competitor to accept a similar contract. But Smith and his top aides said they will not accept any agreement that guarantees fixed levels of employment, even if Ford does.

The UAW demanded fixed levels of employment this week in talks at Ford in nearby Dearborn, Mich. Ford officials are discussing the issue, but the company has yet to make a counter offer to the union's proposal, Stanley J. Surma, Ford's chief negotiator, said today.

"We have floated some ideas. We certainly have to examine these proposals. But the issues are extremely difficult," Surma said.

Smith, however, was less sanguine. "Everybody knows and understands that there are going to be lots of T's and I's different in our contract than the one at Ford, because we are structured differently," Smith said.

A difference between the two firms is components sourcing. GM makes 70 percent of the parts used on its U.S. cars and trucks -- paying component workers the same average $24 an hour in wages and benefits it pays its vehicle assembly workers.

But Ford, which earned $3 billion in the first half of 1987 compared with $1.9 billion for GM in the same period, relies on less-expensive, non-UAW sources for at least 50 percent of its automotive parts.

Put another way, GM employs 120,000 UAW-represented workers in its components operations -- 16,000 more than Ford's entire 104,000-member, UAW-represented work force. A total of 335,000 GM workers are affected by the current talks.

Those differences alone mean that GM cannot offer "a real meaningful amount" of fixed employment in its production operations, Smith said. Also, the boom-and-bust nature of durable goods industries, led by autos, rules out fixed-employment possibilities, Smith said.

"What level of job guarantees are you talking about? If you want to guarantee 10 jobs, that's no problem. But what good does that do for anybody?," Smith said.

The consensus among industry analysts is that foreign competition, overcapacity in the North American market and the need to eliminate aging plants will continue to drive auto employment down.

UAW leaders say they are painfully aware of the job-reduction trend, which is why they say that strong job-security provisions are essential to acceptance of new contracts