Beijing's office overseeing national security matters in Hong Kong on Thursday conducted a joint operation with the local police force’s National Security Department (NSD) to investigate a case involving suspected collusion with foreign forces.

It is believed to be the first time the Office for Safeguarding National Security (OSNS) and the NSD joined forces in a probe.

"In the case, six persons and an organisation were suspected of having committed the offence of 'collusion with a foreign country or with external elements to endanger national security' under Article 29 of the Hong Kong National Security Law between November 2020 and June 2024," the force said in a statement.

The force said the OSNS had, in accordance with relevant legislation, requested assistance and support from the NSD, which arranged interviews with people allegedly involved in the case, and required them to surrender their documents.

The force also searched the residences of the six people and the office of the organisation in Kwai Chung, seizing a number of items including bank documents and devices for further investigation.

"The case is still under investigation, and the NSD will continue to provide assistance and support to the OSNS in accordance with the law," it said.