Chinese police have detained four Japanese citizens suspected of illegal filming inside a military zone in the northern province of Hebei, state media said on Friday, dpa reported.
The four people were under investigation for "having entered a military zone without authorization and illegally videotaped military targets", the official Xinhua news agency quoted provincial state security police as saying.
The state security authorities in the provincial capital, Shijiazhuang, had "taken measures against the four people according to law after receiving a report about their illegal activities", the agency said without elaborating.
"Currently, the case is being investigated," it quoted a police statement as saying.
News of the detention comes amid a growing diplomatic row over Japan's arrest of the captain of a Chinese fishing boat near disputed islands in the East China Sea on September 7.
China has called Japan's arrest of the ship's captain "illegal," but Japan has insisted it is strictly following its law.
In New York on Tuesday, Chinese Premier Wen Jiabao reiterated a demand for the "immediate and unconditional" release of the 41-year-old sailor.
On Wednesday, Japan said it still hoped to hold talks with China over the incident.
Japan accused the Chinese captain of deliberately colliding with Japanese coastguard vessels near the islands, which are known as Senkaku in Japan, Diaoyu in China and Tiaoyutai in Taiwan.
The islands, together with the oil-rich surrounding area, are claimed by all three governments.