Suspects arrested by Beijing's new security office in Hong Kong could be tried on the mainland, a senior Chinese official said on Wednesday, a move certain to spark alarm in the Chinese-ruled city as a contentious new national security law takes effect.

Zhang Xiaoming, executive deputy director of Beijing's Hong Kong and Macau Affairs Office, told reporters during a briefing that the mainland's national security office abides by Chinese law and that Hong Kong's legal system cannot be expected to implement the laws of the mainland.