NEW YORK (CNNMoney.com) -- Some 15,000 members of the United Steelworkers went on strike at 16 Goodyear Tire & Rubber Co. plants in the United States and Canada Thursday after the union rejected the company's latest contract proposal.

The union terminated its contract at 1 p.m. ET Thursday, saying that it could not allow additional plant closures after it helped keep the company afloat through cost-saving measures three years ago.

"The company left us with no option," Steelworkers union executive vice president Ron Hoover said in a prepared statement. "We cannot allow additional plant closures after the sacrifices we made three years ago to help this company survive."

The strike involves 12 plants in the United States and four facilities in Canada, Goodyear said.

"We have 9 union facilities that are still operating, and we will operate with salary workers at effected facilities," a company spokesman told CNN.

The two sides have been in negotiations for about four months.

Goodyear said the steelworkers rejected a proposal that would have improved its competitive position, while maintaining a "substantial commitment" to its North American production base. Its proposals included key items in an agreement the steelworkers reached earlier this year with Michelin's BFGoodrich unit plants in the United States.

Goodyear, which has been shifting North American production to more expensive tires, had proposed protection to all but two of its U.S. unionized plants, while also seeking concessions on health care, pension and productivity issues.

The union had said that it gave Goodyear wage, pension and health care concessions in its 2003 contract and sought plant security and other protections from the latest talks.

"We simply cannot accept a contract that knowingly creates a competitive disadvantage versus our foreign-owned competition and increases our cost disadvantage versus imports," Goodyear chief negotiator Jim Allen said.

Allen said Goodyear remains willing to bargain with the steelworkers and has implemented contingency plans at the plants to try to minimize impact on its customers.

The union is also in contract talks with Bridgestone Corp. covering about 6,000 workers at eight U.S. plants.

Goodyear (down $0.06 to $14.23, Charts) shares were over 2 percent lower in afternoon trade on the New York Stock Exchange, while shares of rivals Bridgestone Corp. (Charts) and Cooper Tire & Rubber Co. (down $0.10 to $9.92, Charts) edged lower

--from staff and wire reports

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