DETROIT—General Motors and Ford are again reducing production at their North American assembly plants due to a shortage of semiconductor chips.

GM said it will extend downtime at eight plants in the United States, Canada and Mexico. Most of the new cuts are two weeks, but production of the Silverado 1500 and Sierra 1500 full-size pickups in Indiana and Mexico will resume Sept. 13 after a week of downtime.

“The situation is still complex and highly fluid, but the team continues to look for creative solutions to minimize the impact on the most demanding and capacity-constrained vehicles,” the company said in a statement.

Ford Motor Co said that it, too, will once again cut production of its highly profitable F-150 pickup truck and two other vehicles. The automaker said that its Oakville Assembly Plant in Canada and Kansas City Assembly Plant in Missouri will be down the week of Aug. 30. Oakville builds the Ford Edge and Lincoln Nautilus crossovers. Kansas City assembles the F-150.

Ford also will cut two of three shifts next week at its Dearborn Truck Plant in Michigan, which produces the F-150.

“Our teams continue making the most of our available semiconductor allocation, finding unique solutions to provide as many high-quality vehicles as possible to our dealers and customers,” the company said in a statement.

The chip shortage has caused rolling shutdowns of automotive assembly plants globally throughout this year. Ford has been hit particularly hard by the lack of chips, losing about 50 percent of its planned production in the second quarter.