Canada's main autoworkers union is making progress in contract talks with General Motors, but some issues are still unresolved, a union spokeswoman said on Monday ahead of a midnight strike deadline.

GM's Canadian arm and the union, Unifor, have been divided over union demands that GM commit to building new vehicle models at its Oshawa, Ontario, plant.

A strike would involve nearly 4,000 workers at two GM plants in the Ontario cities of Oshawa and St Catharines. A third GM plant in Ingersoll, Ontario, is excluded from talks because its 2,700 workers have a separate contract.

"We are making progress to meet and reach a fair settlement," said Unifor spokeswoman Denise Hammond. "We are bargaining on every one of our key priorities, including our top priority, seeking commitments for products and investment."

GM Canada spokeswoman Jennifer Wright said in an email that the company was focused on working with Unifor to reach a "mutually beneficial and competitive new agreement."

Canada has been struggling to get new investments from automakers in its once-thriving vehicle assembly industry, losing out to the Southern United States and lower-cost Mexico.

The union said separately that it had also been fighting over the past few days to hold on to more secure defined benefit pensions plans and seeking higher wages.

Earlier on Monday, Unifor national President Jerry Dias said he was feeling encouraged by the way talks were going.

"We're kicking around all kinds of concepts," he said. "Everything is on the table right now. We're talking about everything."

Dias reiterated his position that the union would not extend its strike deadline.

Without a deal, GM members would have a legal right to strike at midnight (0400 GMT) on Tuesday.

IMPACT UNCERTAIN

The impact of a strike on GM's North American production is uncertain because the automaker has capacity outside Canada to build the same truck engines and transmissions produced at its St Catharines plant.

What is not clear is whether the United Auto Workers, which has said it supports Unifor, would be willing to boost production for GM at its unionized plants in the United States.

A four-year contract covering the workers of GM, Fiat Chrysler Automobiles and Ford Motor Co in Ontario expires on Monday. The union chose GM as its strike target for contract talks. A GM deal would set the pattern the other manufacturers would be expected to follow.

Contract talks could save 2,500 jobs at GM's Oshawa car assembly, or take the plant one step closer to closure. The automaker was already on the verge of shutting one of two assembly lines at its Oshawa plant, with several vehicles either produced in another country or expected to move in 2017.

Flavio Volpe, president of the Automotive Parts Manufacturers' Association, said a strike would be difficult for his Canadian members, but worth it if it means securing the future of GM's Oshawa plant.

A long-term stoppage would be painful, but less painful than a permanent closure," he said.