Electronics giant Apple Inc said on Thursday it has sent independent medical experts to one of its major contractors in China amid accusations that bad working conditions led to several workers' deaths.

"We are deeply sad about the deaths of laborers at Pegatron and have commissioned independent medical professionals from the United States and China to conduct probes since last month," Apple said in a statement.

"While they have found no evidence of any link between the deaths and working conditions there, we realize that is of little comfort to the families who have lost their loved ones."

It added that Apple has a "long-standing commitment to providing a safe and healthy workplace for every worker in our supply chain", and the company has formed a team to work with Pegatron Technology Co at the contractor's plants to ensure that "conditions meet our high standards".

Pegatron, a Taiwan-based manufacturing company that supplies Apple, Sony and Dell, confirmed on Wednesday that four workers at its Shanghai factory, which has nearly 100,000 employees, had died of diseases.

The announcement came after New York-based advocacy group China Labor Watch said on Monday that several workers at Pegatron's Shanghai plant "passed away in a short period of time", Bloomberg reported on Thursday.

Bloomberg cited a statement from the labor rights group as saying that among the dead was a 15-year-old worker who died of pneumonia on Oct 9 at a Shanghai hospital.

The Taiwan company said in a statement that "Pegatron has strict measures in place to verify workers' ages before and after they are hired, and we work with health and safety experts to provide a safe working environment for each and every worker".

The young worker who died in October used his 21-year-old cousin's identification to apply for the job, Pegatron said, adding that the factory did not know he was underage.

This is not the first time Pegatron has been targeted by China Labor Watch.

In July, the organization said it found at least 86 violations at the Pegatron factories in Shanghai and the neighboring city of Suzhou.

The violations included demanding employees work 66-69 hours a week, beyond the legally required 49-hour maximum, without overtime being paid.

Pegatron was also accused of using underage workers as interns who worked the same long hours at the factories.

However, Feng Xiliang, a labor rights expert at Capital University of Economics and Business, told China Daily that many workers at electronics companies are willing to work overtime because they want to earn more money.

"Regular wages at most plants that manufacture electronics products are usually low so those young workers must work longer to earn more," Feng said, adding that the younger generation of migrant workers has stronger awareness of its rights and is calling for a more powerful labor union to protect its interests.

"Consequently, Apple and its suppliers have been improving their performance in the labor rights field because they don't want to project a negative image to consumers."

Wang Kan, a researcher in labor rights at the China Institute of Industrial Relations in Beijing, added that working conditions at Apple's contractors are much better than those at many other electronics factories.

"Compared to the time when Steve Jobs managed the company, Apply now is paying more attention to its image in consumers' eyes when it comes to labor rights," Wang said. "I am not saying it is perfect, but it is fair to say that the company has improved substantially in this regard."

Wang said most underage young people working at manufacturing plants are students sent by vocational schools as interns, and are prone to working with hazards due to loopholes in laws.

"Education laws and regulations do not cover their internships at factories, while labor codes do not categorize them as employees," Wang said.