DETROIT (AP) — Contract talks aimed at ending a 22-day strike by the United Auto Workers against General Motors continued Monday after United Auto Workers union bargainers rejected a company offer on Sunday.

In a letter to members, UAW Vice President Terry Dittes cast doubt on whether there will be a quick settlement in the contract dispute, which sent 49,000 workers to the picket lines on Sept. 16, crippling GM's factories.

Dittes' letter said the union presented a proposal to the company Saturday. He said GM responded Sunday morning by reverting back to an offer that had been rejected and made few changes.

The company's proposal did nothing to address a host of items, Dittes wrote on Sunday, specifying job security for members during the term of the four-year contract.

Normally in contract talks, the union bargains for commitments from the company to build new vehicles, engines, transmissions and other items at U.S. factories represented by the union.

"It did nothing to provide job security during the term of this agreement," Dittes wrote. "We, in this union, could not be more disappointed with General Motors who refuse to recognize the experience and talent of our membership."

In a statement, GM said it continues to negotiate in good faith "with very good proposals that benefit employees today and builds a stronger future for all of us."

The company said it is committed to talking around the clock to resolve the dispute.

Also Monday, Michigan Gov. Gretchen Whitmer said she is concerned about the growing impact of the strike as it entered its fourth week.

Whitmer, who visited workers on the picket lines at a GM plant near the state capital of Lansing, said she hopes the strike ends soon.

"These things are important, that they try to remedy them and find some common ground as quickly as possible," she said.

The strike immediately halted production at GM's U.S. factories. On Monday, GM shut down V8 engine and continuously variable transmission assembly lines at its Ramos Arizpe plant in Mexico, idling about 415 workers. Earlier the company closed two assembly plants in Mexico and Canada.

Despite Dittes' letter both sides continued bargaining Monday. He wrote that progress had been made on important issues last week, but the talks took a "turn for the worse."

Dittes also sent a sharp letter to GM's vice president for labor relations Sunday, saying: "You didn't even have a professional courtesy to explain why you could not accept or why you rejected our package proposal for each item we addressed."

A person briefed on the talks said Sunday that the union voiced concerns about GM increasing production in Mexico, where it now builds pickup trucks, small cars and two SUVs. The person, who spoke on condition of anonymity because the talks are private, said both sides are far apart on guarantees of new products in U.S. factories.

GM leads all companies in automobiles produced in Mexico at just over 833,000 last year, according to LMC and the Center for Automotive Research, a think-tank based in Ann Arbor, Michigan. Of GM vehicles sold in the U.S., 22% are produced in Mexico.

Industry analysts say GM is losing more than $80 million a day as the strike continues. Workers earn $250 per week in strike pay while they're on the picket lines, about one-fifth of what they normally make.