Hong Kong's Apple Daily says a Chinese assistant at the Beijing bureau of Japan's Nihon Keizai Shimbun newspaper was detained by police on Tuesday.
The staffer had frequently interviewed prominent human rights lawyer Pu Zhiqiang, who was detained last week.
Asked about the matter at a regular news briefing on Wednesday, Chinese Foreign Ministry spokeswoman Hua Chunying said she was unaware of it.
The public relations unit of the Nihon Keizai Shimbun said the daily is currently working to confirm the facts.
The Apple Daily also said former South China Morning Post journalist Wu Wei has been taken into custody.