The iPhone and iPad maker has faced pressure to better oversee often-poor manufacturing conditions in China since 13 workers for one of its suppliers committed suicide in 2010.
US-based China Labor Watch said in a report that three plants run by Pegatron Group violated standards set by Apple.
Apple’s website says that these include no underage labour, overtime to be voluntary, and a maximum 60-hour workweek.
China Labor Watch said the 70,000 employees at the three Pegatron sites averaged 66, 67 and 69 hours per week, and that “many workers” were under 18, some of them interns from vocational schools.
Overtime was mandatory during busy periods, it said, adding that employees at one site who refused to work extra hours once would lose the chance to do so for the rest of the month.
The report also described crowded dormitory rooms housing up to 12 people, insufficient fire escape routes and fines for behaviour such as “failing to tuck in one’s chair after eating” and “absence from unpaid meetings”.
It also said managers screened out job applicants who were pregnant or older than 35, and rushed through safety training.
Apple said in a statement it had audited Pegatron facilities 15 times since 2007 and found last month that their workweek averaged 46 hours.
It said it had acted on previous complaints raised by China Labor Watch and would “immediately” investigate claims in the latest report “that are new to us”.
“We are proud of the work we do with our suppliers to uncover problems and improve conditions for workers,” it said.
“Apple is committed to providing safe and fair working conditions throughout our supply chain.”
China Labor Watch said it sent undercover investigators to the three factories and conducted nearly 200 worker interviews between March and July this year.
Pegatron Group, a Taiwanese company, could not immediately be reached for comment.