By interviewing workers and going undercover as employees, New York-based China Labor Watch it had investigated eight factories in China, six of which were Samsung-owned, the other two being suppliers to the company.

In all the factories but one, the group said it found that workers' overtime for at least half of the year reached or exceeded 100 hours per month. In some instances, workers were given only one day off a month. At a Samsung factory in Tianjin that produces mobile displays, overtime can reach 186 hours during months of peak demand, the labor group said.

Four of the factories forced employees to work overtime, China Labor Watch said. But in general, workers at other factories choose to log the extra hours to supplement their low salaries, which can vary from US$200 to $250 a month. With overtime during the busy season, workers can bring their total monthly wages to between $600 and $800.

China Labor Watch on Tuesday, just a day after Samsung said it planned to audit 249 of its suppliers in China in response to earlier criticism from the same watchdog group.