Apple conducted 451 reviews of multiple levels in its supply chain covering facilities where nearly 1.5 million people work, according to its eighth annual internal audit. The company said it's more aggressively trying to remove the use of so- called conflict minerals and has gotten 95 percent of the facilities to keep work to below 60 hours per week.

The report reveals the behind-the-scenes work that goes into making the company's gadgets, from the mining of minerals used in components to assembly lines in China where a final product is put together. Apple sold a record 150.3 million iPhones and 71 million iPads in its latest fiscal year, putting pressure on suppliers such as Foxconn Technology Group to manufacture enough devices fast enough to keep up with demand.

For at least two decades, Apple and other technology companies such as Hewlett-Packard Co. and Dell Inc. have shifted production from the U.S. to Asia, where labor is less expensive and many electronic parts are manufactured. Cupertino, California-based Apple has faced criticism from labor-rights groups for issues at suppliers ranging from excessive overtime to a spate of employee suicides at Foxconn plants.