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Chinese authorities have detained five local staff at the due diligence firm Mintz Group and shuttered its Beijing office.

It is unclear why the US company was raided, but it said it has not received any official legal notice.

The move comes as Beijing prepares to host an annual investor conference this weekend, involving top CEOs such as Apple's Tim Cook and HSBC's Noel Quinn.

Chinese authorities have yet to comment on the incident.

At a regular press briefing, Chinese foreign ministry spokeswoman Mao Ning said she was not aware of the case.

The Mintz Group raid reportedly took place on Monday. The employees, all Chinese nationals, are said to be being held somewhere outside of the Chinese capital.

In a statement, the company said it had retained legal counsel on the matter and requested the release of its staff.

It stressed that it is licensed to conduct legitimate business in China, where it operates "transparently, ethically and in compliance with applicable laws and regulations".

According to its website, the Mintz Group's Beijing office is its only one in mainland China.

The company specialises in background checks, fact gathering and internal investigations. It has 18 offices around the world and hundreds of employees.

Its clients include the National Football League, New York City and The Beatles.

Randal Phillips, a partner at the firm who heads the Mintz Group's Asia operations but is based outside of China, is the former chief representative in China for the Central Intelligence Agency.

While there is no indication that the raid is related to Mr Phillips, he has previously said that the United States should address structural imbalances in trade stemming from Chinese policies.

In 2018, Mr Phillips also testified before Congress on China's efforts to exert international influence.

Relations between Washington and Beijing have been deteriorating in recent months.

Tensions first flared in February after the, which Beijing insists was a weather monitoring device.

On Thursday, about the app's data security and privacy practices, and its alleged ties to Beijing.

According to a survey by the American Chamber of Commerce in China, US businesses operating in China are increasingly pessimistic about their prospects in China, with two-thirds of respondents citing rising U.S.-China tensions as the top business challenge.