U.S. autos companies said yesterday the industry-wide rebates now in effect have been a flop so far, and General Motors announced it was laying off 6,100 workers indefinately.

The five domestic car producers reported daily sales for the first 10 days of February were at their lowest level in 21 years.

GM said it is shutting down its Fremont and South Gate plants in California and eliminating a shift in Pontiac, Mich., "in response to current market conditions and the near-term outlook for new car and truck demands." The closings, which begin next month, indefinately idle 6,100 workers.

GM also said it was recalling another 3,700 laid off workers by adding shifts at three plants to build sportscars and trucks.

In a separate report, United Auto Workers union President Douglas A. Fraser says the union has not "ruled out" the possibility of resuming contract concession talks with General Motors Corp. "But I think the chances of that occurring are certainly a lot less than 50-50," Fraser said on the CBS-TV "Morning" program.

Fraser said the union's historic agreement with Ford Motor Co. to swap concessions for job security signals the "start of a new day" for auto workers.

Currently, more than 246,000 auto workers, including 140,000 at GM alone, are on layoff industry-wide--just 2,000 short of the record 248,000 on layoff last summer.

Industry car sales were down 7 percent on a daily rate basis from last year's depressed levels--even though the auto companies offered cash rebate incentives to stubborn car buyers that ranged to as high as $2,000.

The sales volume was slightly higher this year, but only because there were nine selling days in the period, compared with eight last year.

The daily rate of 15,808 cars was the lowest for that period since 1961 when the daily rate was 13,187.

At that rate auto manufacturers would be selling an annual average of 6.3 million cars.

Industry year-to-date sales are down 15.8 percent to 510,411 from 605,866.

Combined sales by General Motors, Ford, Chrysler Corp. and Volkswagen of America totaled 142,273, compared with 136,032 in the nine-day selling period of 1981.

GM reported sales of 88,131 for the first third of the month, down 0.4 percent on a daily rate basis from the 78,674 sold in the same period last year.

GM sales for the year are off 16 1/2 percent.

Ford reported sales for the period down 14.8 percent. Ford sold 33,824 cars versus 35,294 last year.

The automaker bowed to GM's lead Feb. 2 during since-successful talks with the UAW and announced rebates of $750 to $2,000 on selected models.

Chrysler, which has offered rebates on its cars since the beginning of the year, reported sales of 16,384, down 4 percent on a daily rate basis from 15,241 last year. Chrysler's sales for the year are down 13 percent from 1981.

American Motors had estimated sales of 2,300 for the period, down 41.1 percent from 3,480 in 1981. Year-to-date AMC sales are down 56 percent.

Volkswagen, which offered rebates for the first time ever, saw its sales slump 56.6 percent in the first 10 days of Feburary to 1,634 from 3,345. VW's sales for the year are off 56 percent.