One of four Japanese who were arrested by Chinese authorities last year over alleged involvement in spying activities has been indicted, a Japanese official said.

The man, in his 50s from Aichi Prefecture, was detained last May near a military facility in Zhejiang province in eastern China. He is the first of the four known to have been indicted.

Chinese authorities informed Japan this month that they indicted the man but did not explain what the charges against him were, the official said Monday.

They detained the three men and one woman between May and June last year over alleged involvement in espionage and arrested them on suspicion of acts such as posing a threat to national security.

The two other men are from Hokkaido and Kanagawa Prefecture, and the woman is from Tokyo.

China has been stepping up its watch over foreign organizations and individuals under the leadership of President Xi Jinping, who assumed control of the Communist Party in November 2012.

Chinese authorities have been detaining more foreigners since a counterespionage law took effect in November 2014 and a national security law entered into force last July.