Technology giant Apple has launched a probe into a report that said factories in China that manufactured its products employed child labour and had 24-hour working days and unsafe conditions, a media report said.

In an email sent to around 60,000 employees, the company's chief executive Tim Cook said Apple "cares about every worker in its supply chain", the Telegraph reported.

The letter was reportedly in response to a series of articles in the New York Times that spoke about the company's problems in China.

"Most people would be really disturbed if they saw where their iPhone comes from," an unnamed former Apple executive told the newspaper.

Cook's letter said Apple would "continue to dig deeper" into the reported problems in China. "What we will not do, and never have done, is stand still or turn a blind eye to problems in our supply chain," he wrote.

"Any accident is deeply troubling, and any issue with working conditions is cause for concern. Any suggestion that we don't care is patently false and offensive to us. As you know better than anyone, accusations like these are contrary to our values. It's not who we are," he said.

The Telegraph report said Apple has monitored its factories in China since 2007, but has so far failed to significantly improve working conditions or safety.

Soaring sales of Apple gadgets have put intense pressure on the company's suppliers to ramp up production, leading to a spate of suicides, explosions and poisonings, it said.

In the company's latest report, it found at least 90 factories were asking workers to work for more than 60 hours a week, which were significantly over the 40-hour limit imposed by Chinese law. The company reportedly also found five cases of child labour at factories.