A car is seen turned upside down on Monday in the Foxconn plant in Taiyuan after rioting. [Photo/China News Service]

A Foxconn Technology Group plant in the northern city of Taiyuan, the company making Apple's new iPhone 5, resumed production Tuesday after clashes by workers.

Foxconn and police said the cause of the brawl on Sunday night was under investigation, but labor activists said the rollout of the iPhone 5 had led to longer working hours and more pressure on workers.

Foxconn and police also said as many as 2,000 employees were involved in the brawl and 40 people were injured.

The new phone debuted last week in the United States and eight other markets and Apple has a three to four-week backlog of online orders.

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

News reports and witnesses said the violence in Taiyuan stemmed from a confrontation between a factory worker and a guard that escalated.

One employee said the violence was fueled by workers' anger at mistreatment by Foxconn security guards and managers.

Foxconn did not respond to a request for information on the status of its investigation or whether policies at the factory might change.

Foxconn, which assembles Apple's iPhones and makes components for top global electronics companies, has about 1 million employees on the Chinese mainland.

Labor activists say the need to ramp up iPhone 5 production had increased pressure on Foxconn employees.

Foxconn has declined to say which products are made in each factory but another group, China Labor Watch, said the Taiyuan facility, which employs 79,000 people, is assembling the new iPhone 5.

Foxconn raised minimum pay and promised in March to limit hours after an auditor hired by Apple found Foxconn employees were regularly required to work more than 60 hours a week.

Foxconn's facilities are exceptionally large by the standards of a Chinese electronics industry in which most manufacturers employ hundreds or thousands of workers. Its flagship mainland factory in Shenzhen, near Hong Kong, has 250,000 workers. The Chengdu site has 100,000 and the company has said the Zhengzhou factory might eventually employ 300,000.

Foxconn has also faced criticism in the past over the conduct of its security guards.

In 2010, Foxconn's parent company pledged that its guards would obey the law and refrain from using threats or harassment after a videotape showing several of them beating workers was circulated on the Internet.