A factory in China owned by the manufacturer of Apple's iPhones resumed production Tuesday after a brawl by workers highlighted tensions that labor groups say were worsened by the pressure of a new iPhone launch.

Foxconn Technology Group and police said the cause of the unrest Sunday night was under investigation, but labor activists said the rollout of the iPhone 5 has led to longer working hours and more pressure on workers. Foxconn and police said as many as 2,000 employees were involved in the brawl and 40 people were reported injured.

The iPhone 5 debuted last week in the United States and eight other countries, and Apple has a three- to four-week backlog of online orders.

Foxconn has declined to say whether its suspension of production Monday in Taiyuan in northern China might affect supplies. It did not respond to a request for comment on the labor groups' claims.

News reports and witnesses said the violence Sunday night in Taiyuan stemmed from a confrontation between a factory worker and a guard that escalated. One employee reached by telephone said the violence was fueled by workers' anger about mistreatment by Foxconn security guards and managers.

"Foxconn, some supervisors, and security guards never respect us," said the employee, who asked not to be identified. "We all have this anger toward them, and they [the workers] wanted to destroy things to release this anger."

Production at the Taiyuan factory resumed Tuesday, Foxconn said in a written statement. It did not respond to a request for information on the status of its investigation or whether policies at the factory might be changed.

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Foxconn, owned by Taiwan's Hon Hai Precision Industry Co., is the world's biggest assembler of consumer electronics, with about 1.2 million workers in factories in Taiyuan, Shenzhen in the south, Chengdu in the west, and Zhengzhou in central China.

The Hong Kong group Students & Scholars Against Corporate Misbehaviour, said some employees interviewed at the Zhengzhou factory had not had a day off in 30 days.