Zhu Jianrong, a Chinese scholar based in Japan who comments on Tokyo-Beijing relations is under investigation in China for alleged espionage, Chinese sources said Tuesday.

Zhu, a professor of Chinese politics and diplomacy at Toyo Gakuen University in Tokyo, has been missing since he visited his hometown of Shanghai on July 17, sparking concern about his fate and whereabouts.

According to the sources, the Ministry of State Security suspects the 56-year-old scholar committed illegal acts in connection with his academic research involving Chinese military officers.

When Zhu conducted research in China this year, he interviewed several members of the military in his quest for information on the Chinese navy and other military intelligence matters, an act the authorities may have judged as illegal gathering of information, the sources said.

They suspect Zhu, who moved to Japan in 1986, may not face charges because Chinese authorities sometimes keep influential scholars and activists in custody to "re-educate" them and release them without building a case.

A Chinese person familiar with intelligence activities said Chinese authorities appear to be increasingly sensitive to Zhu's activity amid the deterioration in bilateral relations due to the dispute over the Senkaku Islands, which flared up last September.

There have been rumors that some Chinese government officials appeared to be unhappy with Zhu's comments on Japan-China relations during recent appearances on Japanese TV programs, saying he "stood out too much."

A resident of Chiba Prefecture, Zhu often visits China. He visited Shanghai and elsewhere in February during the Chinese New Year period.

Zhu's family has been unable to contact him, a family member said.