Chinese authorities in September detained a Japanese professor of Hokkaido University, it was learned Friday from sources familiar with bilateral ties.

The professor, who is in his 40s, was taken into custody in Beijing, with the authorities saying he "violated domestic laws," indicating that he may have been suspected of spying, according to the sources.

Since 2015, at least 13 Japanese citizens have been detained in China on various charges including espionage. All of them are civilians, making the latest case involving a national university professor, equivalent to a public servant, the first of its kind.

The professor previously worked for the National Institute for Defense Studies in the Japanese Defense Ministry.

China has been stepping up its watch over foreign organizations and individuals in the name of protecting national security since President Xi Jinping came to power.

Not only Japanese, but a number of other foreign nationals have been held in China, particularly after a counterespionage law took effect in 2014 and a national security law in 2015.