One of four Japanese who were arrested by Chinese authorities between last year and early this year over alleged involvement in spying activities has been indicted, sources privy to Sino-Japan relations said Tuesday.

The man from Kanagawa Prefecture was detained in May last year in Liaoning province, close to China's border with North Korea. He is the third of the four known to have been indicted.

It is not clear what the charges against him were or at which court he will stand trial. The man, a former defector from North Korea, could have been indicted for dealing with sensitive North Korea-related information.

Chinese authorities took the four into custody separately from May to June last year for alleged involvement in spying. China said last month that another Japanese man is under investigation on suspicion of "endangering China's national security."

Japan confirmed the first indictment last May, involving a man in his 50s from Aichi Prefecture, who was detained a year earlier near a military facility in the Chinese coastal city of Wenzhou in Zhejiang province.

Last month, a female executive of a Japanese language school in Tokyo who was detained in Shanghai in June last year became the second of the four known to have been indicted.

Beijing has been tightening its watch over foreign individuals and organizations under President Xi Jinping, with arrests over similar allegations picking up since a counterespionage law came into force in November 2014 and a new national security law took effect in July last year.