Two Japanese men arrested in China last year and charged with unknown offenses are alleged to have violated anti-espionage and national security laws by taking photographs of military facilities, a former high-level Chinese government official revealed Friday.

The information was revealed by a former official of the National Administration of Surveying, Mapping and Geoinformation of China.

The two men and four other Japanese nationals were detained in March last year while they were assisting in geological assessments of sites in Shandong and Hainan provinces after receiving orders from two Chinese hot spring developers.

They were formally arrested in September last year, while the others have already returned to Japan after having been released.

The Global Times, a nationalistic tabloid affiliated with the Chinese Communist Party, had reported in July last year that the two were suspected of having violated the country's anti-espionage and national security laws.

It said authorities retrieved a large amount of classified information, including nearly 80 copies of maps, from the pair's computers and other electronic devices.

The issue of the Japanese detainees was raised at the summit level when Japanese Prime Minister Shinzo Abe visited China this month, and stated that the Japanese government is not engaged in spying activities in China.

The anti-espionage and national security laws were enacted in 2014 and 2015, respectively.