A Chinese court on Friday sentenced a Japanese man in his 70s to five and a half years in prison for stealing state secrets, sources well-informed about Sino-Japanese relations said.

The Intermediate People's Court in northeastern China's Shandong Province also ordered the man, an executive of NC Geophysical Survey Co. in Funabashi, near Tokyo, to forfeit personal assets totaling 30,000 yuan (about $4,300), the sources said.

He was detained in March 2017 in the province while conducting a geological survey with his colleagues. What concrete actions had been deemed illegal or other details of the case remain unknown, the sources said.

Chinese authorities have indicted at least nine Japanese nationals on suspicion of espionage since 2015. The verdict was the sixth to be delivered among the nine cases.