DETROIT (Reuters) - Talks for a new four-year labor contract between General Motors Co (GM.N ) and its striking workers took a turn for the worse on Sunday after the United Auto Workers rejected the largest U.S. automakers latest offer but the two sides were still talking.

GM made an offer to the union that basically repeated one the UAW had previously rejected, Terry Dittes, the UAW vice president in charge of the GM department, said in a letter to members. The union provided a copy of the letter to Reuters.

These negotiations have taken a turn for the worse, he said.

We, in this union, could not be more disappointed with General Motors, Dittes said. The company has shown an unwillingness to fairly compensate ... the UAW.

Dittes said the UAW made an offer on Saturday to GM that covered wages, signing bonuses, job security, profit sharing and other issues. He said GM responded on Sunday morning with its counteroffer, which did nothing to advance a whole host of issues.

GM said in a statement that it was committed to negotiating around the clock to reach a deal.

We continue to negotiate in good faith with very good proposals that benefit employees today and build a stronger future for all of us, the company said in the statement.