A strike at an auto parts supplier has forced General Motors Corp. to shut down three more pickup truck factories, idling 12,000 workers, the company said Friday.

Plants in Flint; Fort Wayne, Ind.; and Oshawa, Ontario, were to close at the end of the second shift on Friday night, GM spokesman Tom Wickham said.

The closures are in addition to the idling of a pickup truck factory in Pontiac on Thursday. All the plants make the Chevrolet Silverado and GMC Sierra full-sized pickups.

It is all due to a strike against parts supplier American Axle and Manufacturing Holdings Inc. The Detroit-based company makes axles and other parts for the trucks, and GM is running out of the parts, Wickham said.

He would not say if the strike could lead to more GM plant closures, but industry analysts have said American Axle also makes components for GM's large sport utility vehicles.

"We are monitoring supply to other facilities," Wickham said.

Factories in Janesville, Wis., and Arlington, Texas, that make the large SUVs still were operating on Friday. Their vehicles include the Chevrolet Tahoe and Suburban, GMC Yukon and Cadillac Escalade.

GM also still was making pickups and large SUVs at a factory in Silao, Mexico, and it makes a small number of pickups at a plant in Toluca, Mexico.

About 3,600 workers represented by the United Auto Workers at five American Axle plants in Michigan and New York went on strike early Tuesday in a contract dispute. The strike continued Friday. As of Thursday night, no talks had been scheduled.

American Axle had been using stockpiled parts to keep supplying GM and its other customers, although it appears those supplies are running out. The company makes axles, drive shafts and stabilizer bars.

GM accounts for about 80 percent of American Axle's business, with 10 percent going to Chrysler LLC and the rest to other automakers.

GM shares fell 64 cents, or 2.6 percent, to $23.86 in afternoon trading Friday.