Japan confirmed Monday that a Japanese man in his 40s has been detained by Chinese authorities since September on suspicion of violating Chinese law, saying it has been providing him with necessary assistance.

“We have extended our maximum support, with consular officials meeting him and helping him get in touch with his family,” Chief Cabinet Secretary Yoshihide Suga said at a news conference.

The top government spokesman did not give further details, citing the sensitivity of the issue.

Sources familiar with bilateral ties said Friday that Chinese authorities detained in September a male professor from Hokkaido University, indicating that he may have been suspected of spying. The professor at the national university had worked in the past for the National Institute for Defense Studies within the 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 in 2013.

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Since 2015, at least 13 Japanese citizens have been detained in China on various charges including espionage. All of those previously arrested were from the private sector.