Taiwan's Foxconn has admitted employing children as young as 14 on assembly lines at a plant in China, a fresh blow to the tech giant that has been attacked over its treatment of staff after several suicides.

The company, which makes products for Apple and Sony, admitted it hired the underage workers as part of an internship programme, reflecting a practice rights groups said is widespread among enterprises in China.

"This is not only a violation of China's labour law, it is also a violation of Foxconn policy," the company said in a statement late Tuesday, referring to Chinese rules that set the legal minimum age for workers at 16.

"Immediate steps have been taken to return the interns in question to their educational institutions," it added.

Foxconn, the world's biggest contract manufacturer, said it had carried out a probe at the plant in eastern Shandong province, which showed the interns in question, aged from 14 to 16, had worked in there for about three weeks.

A spokesman told AFP the interns had been mainly working on assembly lines at the plant.

"We have found no evidence of similar violations in any of our other campuses in China but we will not hesitate to take immediate action in any campus if any violations are discovered," it said.

The company issued the statement after Chinese media and US-based China Labor Watch reported on the use of underage workers at the plant.

Foxconn said it has long had a short-term internship programme carried out in cooperation with vocational schools and other educational institutions in China.

Chinese government regulations bar vocational schools from sending first-year students on internships, but they have often violated these rules, another group, China Labour Bulletin, said in a report earlier this year.

"Given that first year students could be just 15 years old, such factories would technically be employing child labour in some cases," the report said.

Geoffrey Crothall, a spokesman for China Labour Bulletin, said interns formed a "cheap and convenient source of labour" that vocational schools are only too happy to provide as it helps boost their revenues.

"The enterprises tend to be factories that need more hands on the production line. There is no real training or apprenticeship involved here," he said.

Interns represent approximately 2.7 percent of Foxconn's workforce of 1.2 million employees in China, the company said, but Crothall said he thought the number looked too low.

The situation in China now is comparable to the situation in neighbouring economies a few decades ago, including Taiwan, which issued rigid rules banning the employment of workers under 15 in 1984, observers said.

"China is repeating what happened in Taiwan in the 1970s and 1980s when the economy was taking off and needed a lot of labour," said Chu Wei-li, a spokesman for Taiwan's National Federation of Independent Trade Unions.

The news is the latest in a string of problems to beset Foxconn, which has frequently been targeted for its labour practices following a spate of suicides in 2010 that activists blamed on tough working conditions, prompting calls for better treatment of staff.

And earlier this month it was hit by two labour disputes that saw thousands of people go on strike, which China Labour Watch said was over increased product quality levels and demands that staff work through a national holiday.

Foxconn is the world's largest maker of computer components and assembles products for Apple and Sony as well as Intel and Nokia, among others.

Hong Kong-listed shares in Foxconn fell 1.35 percent to HK$2.92, while the Hang Seng index rose 0.94 percent in the afternoon.

In Taipei, shares in Foxconn's parent company Hon Hai Precision fell 0.23 percent to Tw$87.4, against a drop in the broader index of 0.09 percent.