Workers at a Foxconn Group factory in China that assembles the iPhone 5 for Apple have gone on strike following disputes over quality control, said the labour rights group China Labour Watch.
The group claimed that at 3,000 to 4,000 workers had walked out, alleging that, "Foxconn raised overly strict demands on product quality without providing worker training for the corresponding skills".
The company is also accused of making employees work during holidays.
The tough working conditions have resulted in falling output quality, triggering disputes between quality control inspectors and the workers. This has put more pressure on the workers, said China Labour Watch.
A fight between quality control inspectors and workers is reported to have broken out in the plant, causing property damage and injury.
The latest strike has left a number of iPhone 5 production lines dormant, said China Labour Watch. It is not clear how long the strike is likely to last.
Foxconn is the major supplier of Apple's iPad and iPhone. It employs around 1.2 million workers in around 18 countries, including China, Taiwan, Mexico and Brazil.
Apple is already reported to be struggling to meet demand for its recently released smartphone. The strike puts further pressure on the US firm as it looks to roll out the product globally.
The latest walkout is the latest in a series of problems to hit the Taiwanese company. In August, another Foxconn plant was forced to suspend operations following tension between employees and management.
"What's important is the implication," Daniel Chang, an analyst with Macquarie Securities in Taipei, told Bloomberg.
"At a time when China's wage level is rising it's creating big challenges for assembly plants like Foxconn."
Apple refused to comment on the strike. But Foxconn told Reuters that it was "gathering facts" on the matter and will issue a release when ready.