TORONTO (AP) — Auto workers walked off the job at three General Motors facilities in Canada early Tuesday after failing to reach agreement with the automaker.

Their union, Unifor, represents more than 4,200 workers at the plants. They had warned they would begin a strike if no agreement was struck with GM by midnight local time.

“We made some progress throughout the day, but sadly not enough,” Unifor President Lana Payne told reporters. She said the union was still speaking with the company, but there was “a lot of ground that needed to be covered to reach a tentative agreement.”

The action came after Unifor workers ratified a new three-year labor contract with Ford late last month. They are seeking a similar agreement with GM.

“This strike is about General Motors stubbornly refusing to meet the pattern agreement. The company knows our members will never let GM break our pattern — not today — not ever,” Payne said.

She said GM was not meeting the union’s demands for pensions, support for retired workers and steps to transition temporary workers to permanent, full-time jobs.

“We are not there yet, so as a result we are on picket lines,” Payne said.

General Motors Corp. said that while “very positive progress” had been made, the company was disappointed not to be able to win an agreement.

“We remain at the bargaining table and are committed to keep working with Unifor to reach an agreement that is fair and flexible for our 4,200 represented employees at Oshawa Assembly & Operations, St. Catharines Propulsion Plant, and Woodstock Parts Distribution Centre,” Jennifer Wright, GM Canada’s executive director for communications, said in a statement.

Payne said earlier that the union had a lot of bargaining leverage with GM because the factory in Oshawa, Ontario, is working around the clock to build profitable Chevrolet pickups. However, in her remarks to reporters she said “demographics,” presumably of an aging work force, were a major hurdle.

Workers at Ford of Canada ratified a new deal late last month that raises base hourly pay for production workers by almost 20% over three years.

Unifor had earlier avoided going on strike against the Detroit automakers, unlike its U.S. counterpart, the United Auto Workers.

Its members at a fourth GM facility, the CAMI Assembly Plant in Ingersoll, Ontario, are covered by a separate bargaining agreement and remain at work, the Unifor statement said.

Unifor is Canada’s largest private sector union, with 315,000 workers in many industries.