China last month arrested professor Hokkaido University, Japan, on suspicion of making a message but the incident was revealed yesterday.

Professor in his 40s detained in Beijing. Officials say he "violated domestic law", proving he could be arrested for espionage, according to a source.

This was the first case of suspicion of espionage involving a professor, the equivalent of an official. Since 2015, at least 13 Japanese citizens have been arrested in China on various charges, including espionage. They are all civilians.

Professor was forced to work for the National Defense Research Institute under the Ministry of Defense of Japan.

Many foreigners have been arrested in China on suspicion of espionage, especially when its anti-espionage law took effect in 2014 and the national security law has been in place since 2015.