BEIJING, China — A factory in China owned by the manufacturer of Apple’s iPhones resumed production yesterday after a brawl by workers highlighted tensions that labour 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 labour 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 one-day suspension of production Monday in Taiyuan might affect supplies. It did not respond to a request for comment on the labour groups’ claims.

News reports and witnesses said the violence Sunday night in Taiyuan in northern China stemmed from a confrontation between a factory worker and a guard that escalated. One employee reached by telephone said the violence was fuelled 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 by name. “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 yesterday, 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.

Foxconn, owned by Taiwan’s Hon Hai Precision Industry, is the world’s biggest assembler of consumer electronics, with about 1.2 million workers in factories in Taiyuan, the southern city of Shenzhen, in Chengdu in the west and in Zhengzhou in central China. It makes iPhones and iPads for Apple and also assembles products for Microsoft and Hewlett-Packard.

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

“Because of the launch of the iPhone 5, the workload of workers suddenly surges,” said a Hong Kong group, Students & Scholars Against Corporate Misbehaviour, in a report this month. It said some employees interviewed at the Zhengzhou factory had not had a day off in the previous 30 days.

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 making the iPhone 5.

The group, based in New York City, complained that employees suffer “verbal and physical abuse” by guards.