Many companies are having trouble following minerals found in their products, from cell phones to cars, that can fuel conflict in central Africa.

An estimated 61% of companies reported to regulators in 2018 that they couldn’t confirm where the minerals came from, according to U.S. government research issued Sept. 9. That’s up from 47% in 2017 and 55% in 2016.

Companies cited difficulties accessing suppliers and tracking the complex chain from mines to processing facilities, according to the study from the U.S. Government Accountability Office.

Public companies that use these so-called conflict minerals—tin, tantalum, tungsten, and gold and others mined...