Oct 10 (Reuters) – About 4,300 unionized workers went on strike at three General Motors (GM.N) plants in Canada on Tuesday, boosting pressure on the automaker grappling with a U.S. union work stoppage now in its fourth week.

The walkout by workers came after Canadian union Unifor said GM was “stubbornly refusing” to match the contract the labor union reached with Ford Motor (F.N), which offered wage increases of up to 25% in Canada.

“The company continues to fall short on our pension demands, income supports for retired workers, and meaningful steps to transition temporary workers into permanent, full-time jobs,” Unifor National President Lana Payne said.

The union had set a deadline of Monday midnight for a new deal with GM after the previous contracts with the Detroit Three automakers expired on Sept. 18.

GM said it would continue talks with Unifor. The walkout adds to the headache faced by the automaker in the U.S. where it is racking up millions of dollars in losses per week due to the United Auto Workers (UAW) strike that started Sept. 15.

GM has lost 34,176 vehicles of production since the start of the UAW strike, according to an estimate by Deutsche Bank. The automaker said last week it had 442,586 vehicles in stock.

The UAW has struck two GM assembly plants in the United States and 18 parts distribution centers. GM has laid off 2,300 U.S. workers due to the impacts of the UAW strike.

Unifor said it would go on strike at GM’s Oshawa assembly complex, St. Catharines powertrain plant and the Woodstock parts distribution center, but members at the CAMI Assembly Plant in Ingersoll, Ontario, will work as they are covered by a separate agreement.

GM now faces a likely disruption in production as workers at the St. Catharines plant make engines for a variety of vehicles, powertrains for the Chevrolet Equinox and Corvette, as well as engine component parts.

At the Oshawa plant, workers build Chevrolet Silverado trucks, one of GM’s most profitable models, while the plant’s stamping operations supply various parts for GM North America. GM did not immediately say Tuesday when it expects disruptions from the Canadian strike to affect U.S. auto production.

Wells Fargo said in a research note that Oshawa was the smallest of GM’s pickup plants, producing about 2,800 trucks per week, but added the impact was likely wider at St. Catharines since the “majority of GM’s large SUVs and heavy-duty full-size pickups use the V8 engines. Also, about half of standard full-size pickups use V8 engines, so engine options on these vehicles could be limited if the strike drags on.”

Unifor has used the “pattern bargaining” approach in its talks, reaching a deal first with Ford and then expecting GM and Stellantis (STLAM.MI) to match. The UAW, on the other hand, broke with that approach under its new leadership.

Unifor represents about 18,000 Canadian workers at Ford, GM and Chrysler parent Stellantis.

Unions have been increasingly resorting to strikes across sectors from airlines to automakers, buoyed by a tight labor market and positive public opinion in the U.S., even though union membership has fallen.

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