Talks between the United Auto Workers (USAW) union and General Motors "have taken a turn for the worse," the union's chief negotiator has said, suggesting that no immediate end was in sight for the US vehicle industry's longest strike in decades.

The setback in talks followed several days of reported progress between the two sides, CNN reported.

CNN said Terry Dittes, vice president of the United Auto Workers union negotiating team, sent a letter to members at mid day on Sunday saying the union found the company's latest contract proposal to be totally unacceptable.

"They reverted back to their last rejected proposal and made little change," Dittes' letter said, according to CNN. He said the union negotiators "could not be more disappointed" in GM's position.

The strike is now in its 21st day.

"We continue to negotiate in good faith with very good proposals that benefit employees today and builds a stronger future for all of us," GM said in a statement cited by CNN.

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"We are committed to continuing discussions around the clock to reach a resolution."

The union wants GM to bring some production in Mexico to the United States. But a source familiar with the negotiations told CNN bringing back a product lines from Mexico still has not been addressed in the talks.

Dittes' letter reportedly said GM's proposal did nothing to provide the job security guarantees the union is demanding.

Additional issues include wages and benefits and profit sharing.

"The company has shown an unwillingness to fairly compensate the great workforce of the UAW," said Dittes' letter.

CNN said union contracts with Fiat Chrysler and had been extended while the union tried to reach a deal with GM.

In contrast, the UAW said earlier "significant progress" had been achieved in contract talks with Ford.

The update from the union indicated 18 out of 20 Ford subcommittees had reached tentative agreements or made significant progress, but didn't break down the remaining sticking points.