Beijing is investigating a US due diligence firm for "illegal" activities, a government spokesperson said on Monday, after the company said five local employees were detained.

The New York-based Mintz Group is "suspected of illegal operations", Foreign Ministry spokesperson Mao Ning said, adding that "the relevant case is now under investigation".

Mao did not share further details about the investigation into the group, which specialises in investigations into fraud, corruption and workplace misconduct allegations, as well as background checks.

In an emailed statement to Reuters last week, Mintz said: "We can confirm that Chinese authorities have detained the five staff in Mintz Group's Beijing office, all of them Chinese nationals, and have closed our operations there."

It said it was ready to work with mainland authorities to "resolve any misunderstanding that may have led to these events", and that its top concern was the safety and wellbeing of colleagues in China. (AFP/Reuters)