Chinese officials have closed down the Beijing offices of U.S. due diligence firm Mintz Group and detained five of its employees.

The employees are all Chinese nationals.

In a statement sent to Reuters, 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 a multinational company with 18 offices, including in Washington, that conducts investigations and background checks.

The closing of its Chinese office comes at a tense time in U.S.- Chinese relations. Last month, the United States shot down what it says was a Chinese spy balloon over U.S. territory. China insists the balloon was a weather monitoring device.