New York-based China Labor Watch said in a report that Pegatron Corp was guilty of safety and environmental violations, and living conditions for workers at its factories in China were poor.
(Also see: Apple supplier Pegatron accused of labor abuses)
Pegatron pledged an in-depth probe.
"We believe our employees are the core of the company. Therefore, we've been seriously looking at all labour-related issues and will conduct a thorough investigation," CEO Jason Cheng said in a statement.
Cheng said Pegatron would make immediate improvements if the investigation found any irregularities that violate China's labour law and the company's code of conduct.
"We've also been checking related issues so as to ensure that our own standards and those of clients are met," his statement said.
The US rights groups said three Pegatron plants in China impose excessive overtime and employ minors. It also cited crowded dormitories, insufficient fire escape routes and arbitrary fines for perceived minor lapses of behaviour.
Apple has also said it will investigate the claims. It said in a statement it was "committed to providing safe and fair working conditions throughout our supply chain".
Apple said it had conducted 15 comprehensive audits at Pegatron facilities since 2007, including surprise audits within the past 18 months.
Pegatron produces consumer electronics like game consoles, television sets and computers for Sony, Toshiba and some other brand name vendors, as well as assembling products for Apple, an official of the Taiwanese company said.
It currently has around 110,000 employees, the vast majority of them in the Chinese cities of Shanghai, Suzhou and Chongqing.
Dow Jones Newswires said that since last year, Apple has been shifting orders from another Taiwanese assembler Hon Hai Precision to Pegatron for the iPad Mini and for an anticipated cheaper iPhone this year.
Hon Hai's China unit Foxconn has come under the spotlight in recent years over worker suicides, labour unrest and the use of underage interns at its Chinese plants. It has taken steps to address concerns.
Foxconn is the world's largest contract electronics maker and assembles products for Apple, Sony and Nokia, among others.
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