General Motors will lay off 900 factory workers when it eliminates the third production shift at its Pontiac truck assembly plant in January, the Detroit News reported.

The workers will be laid off, offered transfers to other company plants or given the option of retiring, if eligible, GM reportedly said.

The paper said the move comes as GM steps up cost-cutting efforts to reverse an unexpected loss in its North American automotive business. Because of slow sales, GM also idled a Saturn plant in Spring Hill, Tennessee, that makes the Ion.

The Detroit News said the Pontiac plant employs about 3,500 workers and is one of four North American factories that build the Chevrolet Silverado and GMC Sierra full-size pickups.

"This was a market-driven decision," GM spokesman Dan Flores told the paper. "It's better to align the plant's output with current and forecast market demand for the products built in Pontiac."

Though demand for the Sierra is up 9.9% this year, and sales of the Silverado have climbed 1.9% compared with last year, the bulk of sales have been for crew cab models that are not produced at the Pontiac plant, the report said.

The paper said GM has once again idled its Saturn Ion small-car assembly line, despite plans to spend nearly $3 billion to almost double sales at the division in the next three years. The two-week shutdown of the Ion model production line comes about a month after the company reduced the Ion assembly team from three shifts to two. Sales reportedly have failed to meet expectations.