Taiwan's Mainland Affairs Council -- its top China policy agency -- said Friday that public security authorities in the central Chinese city of Zhengzhou had held the employees for the equivalent of "breach of trust" violations in Taiwanese law.

"The circumstances surrounding this case are quite strange," the council said, adding that Foxconn had declared that the staff members had not caused the company to suffer losses or damage to its interests.

Beijing's state news agency Xinhua on Saturday then cited the mainland's Taiwan Affairs Office as saying that the group were being "investigated for suspected crimes of taking bribes and embezzlement".

Office spokesperson Zhu Fenglian said "relevant authorities will investigate the case in accordance with the law and ensure the legitimate rights and interests of the suspects are well protected", according to Xinhua.

The report did not give details of the allegations or the identities of those under investigation.

Foxconn, also known by its official name Hon Hai Precision Industry, is the world's biggest contract electronics manufacturer and assembles devices for major tech companies, including Apple.

Most of its factories are in China, including Zhengzhou, which is dubbed "iPhone City" as the home of the world's biggest factory for the smartphone.

The Straits Exchange Foundation, a semi-official body in Taiwan handling people and business exchanges with China, told AFP on Friday that the four detainees were Taiwanese.

Taipei's Mainland Affairs Council said the case "may involve corruption and abuse of power by a small number of public security officials, which has severely damaged business confidence".

It urged Chinese authorities to "investigate and address the matter promptly."

A Foxconn spokesperson declined to comment when contacted by AFP on Friday.

China claims Taiwan as part of its territory and has refused to rule out the use of force to seize control of the self-ruled island one day.

Many Taiwanese companies have set up factories in China during the country's economic opening over the last four decades, but investment has fallen sharply in recent years owing to regional tech disputes.

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HON HAI PRECISION INDUSTRY (FOXCONN)

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