After 20 straight days of negotiations, the United Auto Workers union said it would strike General Motors Corp. Monday morning if a new contract agreement isn't reached, citing the automaker's failure to address job security and other concerns.

"We're shocked and disappointed that General Motors has failed to recognize and appreciate what our membership has contributed during the past four years," UAW President Ron Gettelfinger said in a statement early Monday.

Gettelfinger didn't offer specifics, but the UAW had been expected to ask GM for guarantees of future production at U.S. plants as part of the negotiations.

The union set the strike deadline for 11 a.m. EDT Monday. Cal Rapson, the UAW's chief GM negotiator, said the union will remain at the bargaining table until the deadline. He said GM has failed to meet the needs and concerns of the UAW's members.

"Instead, in 2007 company executives continued to award themselves bonuses while demanding that our members accept a reduced standard of living," Rapson said in a statement. "The company's disregard for our members has forced our bargaining committee to take this course of action."

GM spokesman Dan Flores said the automaker is working with the union to resolve issues.

"The contract talks involve complex, difficult issues that affect the job security of our U.S. work force and the long term viability of the company," Flores said. "We are fully committed to working with the UAW to develop solutions together to address the competitive challenges facing General Motors. We will continue focusing our efforts on reaching an agreement as soon as possible."

The union may be trying to pressure GM to get a deal. UAW President Ron Gettelfinger said as recently as Friday that the union was trying to speed up negotiations and didn't want to strike.

"It is our desire to reach an agreement without a strike, and we have demonstrated this by staying at the bargaining table up to this point," Gettelfinger said in a memo to local union leaders that was posted on a union Web site in Oklahoma.

Harley Shaiken, a professor at the University of California at Berkeley who specializes in labor issues and has closely followed the talks, said the strike threat is a tool to speed the talks.

"I have a feeling it is meant to resolve the issues as quickly as possible," Shaiken said.

Still, he said the threat should be taken seriously. Shaiken said the union wouldn't set a deadline if it thought there was no possibility of a strike.

The UAW hasn't called a nationwide strike during contract negotiations since 1976, when Ford Motor Co. plants were shut down. There were strikes at two GM plants during contract negotiations in 1996.

The UAW currently represents 73,000 GM workers at 82 U.S. facilities nationwide, including assembly and parts plants and warehouses. If workers go on strike, they will be paid $200 a week plus medical benefits from the UAW's strike fund. The union had more than $800 million in the fund as of last November, according to the UAW's Web site.

The UAW's original deadline to reach an agreement with GM was Sept. 14, but the union decided to extend the contract on an hour-by-hour basis and keep talking. Throughout the week, local union officials were told to keep workers ready to strike if necessary, but it was widely believed that the negotiators were making progress and a strike wouldn't be called.

A local UAW official said Sunday that negotiators had wrapped up work on most issues and were determining how much money GM must put into a trust fund for retiree health care that will be managed by the UAW. The official spoke on the condition of anonymity because the talks are private.

The health care fund — known as a Voluntary Employees Beneficiary Association, or VEBA — would be a significant change for the auto industry and has been the major issue in this year's negotiations. GM has around $51 billion in unfunded retiree health care costs but the company isn't required to put the full amount into the VEBA. The UAW and GM have been wrangling over how much GM should put in and how much can be paid in cash or in stock.

GM, which has about 339,000 UAW retirees and spouses, badly wants to pay the union to form the VEBA to get the health care liabilities off its books. In exchange, the UAW was expected to ask for future work guarantees at its U.S. plants.

The UAW picked GM as the lead company and potential strike target in the negotiations, which began in July. Ford Motor Co. and Chrysler LLC have indefinitely extended their contracts with the union. They are expected to match many of the terms of GM's agreement once it's reached. The three automakers have a total of $90 billion in unfunded retiree health care costs.

If a tentative agreement is reached, local union leaders will meet for a briefing and then present the contract to their members. Any agreement would have to be ratified by a majority of GM's UAW members.