Audits of Chinese factories by the U.S. companies hiring them to make products are often "corrupt," with auditors taking bribes to ignore problems, Li Qiang, executive director of China Labor Watch, told the on Tuesday. The audit system used by many U.S. companies to check on working conditions in Chinese factories is "severely flawed," he said.

Factories used by Hong Kong-based cordless phone maker VTech Holdings require workers to work 12- to 15-hour shifts and stand all day, said Charles Kernaghan, executive director for the Institute for Global Labour and Human Rights. Employees on the assembly line are required to add 1,600 pieces to circuit boards in an hour, and live in filthy and crowded dormitories, Kernaghan told the committee. Workers who don't meet production goals work without pay until they can, he said.

"The production line never stops," Kernaghan said. "The pace is relentless, furious, mind-numbing, exhausting."

VTech disputed recent reports from the institute. The company is considering legal action against Kernaghan's group, VTech said in a statement.

"VTech is a responsible and caring employer wherever it has operations, and this includes mainland China," the company said. "The group and its subsidiaries abide strictly by the legal requirements relating to employment in all jurisdictions where it operates, including mainland China."