BEIJING (AP) — A Chinese factory owned by the manufacturer of Apple's iPhones has resumed production after a brawl involving 2,000 workers highlighted chronic labor tensions in a country that prohibits independent unions.

Foxconn Technology Group said the cause of the unrest Sunday night was under investigation, but it comes amid a series of protests by workers in areas throughout China over grievances about pay and working conditions.

News reports and witnesses said the violence Sunday stemmed from a confrontation between a factory worker and a guard that quickly escalated. One employee said the violence was fueled by anger among factory workers over treatment by Foxconn security guards and managers.

Production at the Taiyuan factory resumed on Tuesday, according to an employee who answered the phone at the factory's labor office.