Chinese authorities have detained five local employees of US due diligence firm Mintz Group and shut down the company's Beijing office, the group said Friday.

The company specialises in conducting investigations into fraud, corruption and workplace misconduct allegations, as well as background checks.

"Chinese authorities have detained the five staff in Mintz Group's Beijing office, all of them Chinese nationals, and have closed our operations there," the company said in a statement emailed to AFP.

The firm has "retained legal counsel to engage with the authorities and support our people and their families", it added.

Mintz said it has not "received any official legal notice regarding a case against the company and has requested that the authorities release its employees".

"Mintz Group is licensed to conduct legitimate business in China, where we have always operated transparently, ethically and in compliance with applicable laws and regulations," it added, saying it would work with authorities to "resolve any misunderstanding that may have led to these events".

Visited by AFP journalists Friday, the firm's Beijing office was void of any activity, with the glass front doors firmly sealed with a chain.

Police stations in the area refused interview requests. China's foreign ministry said it was not aware of the reports.

Mintz Group has offices in 18 locations including Washington.

The firm's website says that it digs "deeply into factual questions that concern our clients -- from the presidential palace to the offshore oil rig".

Mintz Group's Asia head Randal Phillips said in 2017 that the United States should address structural imbalances in trade stemming from Chinese policies.

A page on the company's website titled "China must face some consequences" featuring the Phillips quote appeared to have been deleted, though cached versions remained online.

Phillips also testified before the congressional US-China Economic and Security Review Commission in 2018 on China's efforts to exert international influence, according to US government documents available online.

A personal page on the Mintz website describes him as having spent 28 years with the US Central Intelligence Agency's (CIA) National Clandestine Service, "most recently serving as the Chief CIA representative in China".

The detentions come in the face of some of the worst US-China relations in decades, as the two powers clash over everything from trade to human rights.

Tensions flared in February after the United States shot down an alleged Chinese spy balloon, which Beijing insisted was a weather monitoring device.