Auto parts maker Dana Corp. plans to close eight North American plants and reduce operations at three others, the company said Thursday in a regulatory filing.

Dana, which filed for bankruptcy protection in March, said it would cut health benefits for retirees and attempt to alter labor contracts at its unionized plants.

"Our existing labor costs, especially in the U.S., impair our financial position and are a significant impediment to a successful reorganization," the company said in the filing with the U.S. Securities and Exchange Commission.

It was not immediately clear what plants would be closed or how many jobs would be affected.

Dana, which sells brakes, axles and other parts to most major automakers, said in its bankruptcy filing that rising energy costs were driving up production costs and hurting demand for its customers' products.

A large slice of its business comes from three customers: Ford Motor Co. (F), General Motors Corp. (GM) and DaimlerChrysler AG (DCX).

The company already has started to shift operations at two of its plants and close four U.S. sealing and thermal plants and one in Canada.

"We expect to continue to move manufacturing capacity from the U.S. to lower cost countries, such as Mexico," the company's filing said.