No one has so far been arrested in the first known joint operation between Beijing's office overseeing national security matters in Hong Kong and the local police force, Secretary for Security Chris Tang said on Saturday.

The Office for Safeguarding National Security had requested assistance from the force's National Security Department for a probe into suspected collusion with foreign forces, including by arranging interviews with people involved in the case.

"Those who were being interviewed could be suspects, or witnesses, or someone who could provide information. I want to emphasise that we have not arrested anyone so far," Tang told a Commercial Radio programme.

The minister also noted that Beijing authorities were empowered under the city's home-grown security legislation to ask SAR officers for support, to determine if a situation was one of three exceptional circumstances laid out in Article 55 of the Hong Kong National Security Law.

Article 55 of the law stipulates that the Office for Safeguarding National Security can exercise jurisdiction if a case is determined to be complex due to the involvement of a foreign country or external elements, or is considered a "serious situation" where the SAR government is unable to enforce to law effectively, or counter a "major and imminent threat to national security".

"This time, we have helped to arrange some interviews. Under the current circumstances where Article 55 has not been invoked, the Hong Kong police are responsible for carrying out law enforcement," Tang said.

"In order to determine whether there is such a situation, Beijing's office relied on our legal assistance, which was normal and reasonable."

National security police recently warned that the mobile app "Reversed Front: Bonfire" has been deemed to be seditious and those who have downloaded it or provided funds to its developer may be in breach of the security legislation.

The security chief said the app had shown the "viciousness of soft resistance", adding that it aimed to "corrupt the minds of young people".