BEIJING (Kyodo) -- China said Monday six Japanese nationals have been detained for their suspected involvement in "illegal activities," without elaborating on the exact allegations or other details.
The confirmation of their status came after a Japanese government source first revealed that the six, all men, were detained in March, fueling speculation that they could have been held on suspicion of endangering national security, the allegation China increasingly applies to foreigners.
Japan's top government spokesman Yoshihide Suga told a press conference in Tokyo that officials are providing "necessary assistance" to the detainees from a standpoint of protecting Japanese nationals.
Suga, however, said the government cannot provide other information considering the "nature" of the matter.
Four of the men, who are in their 20s to 70s, are working for NC Geophysical Survey Co., a Japanese company based in Funabashi, a city located about 20 kilometers east of Tokyo, and two others are from different firms.
Three of the men were detained in Shandong Province and the other three in Hainan Province, according to the company, which said it believes they did not do any wrongdoing and it was told by the Japanese Foreign Ministry that they are in good shape.
The firm said it received orders from two Chinese hot spring developers to assist geological assessments of a site in Penglai in the eastern province of Shandong and another location in Wuzhishan in the southern island province of Hainan.
The detainees started doing their work from late March in the two provinces, NC Geophysical Survey said, adding the assessments were due to complete in early April, but it lost contact with them in the middle of the schedule.
The company with about 50 employees said the four had worked in China several times before. It also said that two Chinese nationals were held along with the Japanese in the city in Hainan.
China's Foreign Ministry spokeswoman Hua Chunying, speaking at a regular press briefing, said they are under investigation in accordance with China's law.
It is unclear why they are being held, but Chinese authorities may suspect them of stealing national secrets as there were cases in the past that foreigners were held for collecting geographical data.
A different source familiar with the matter said China's state security officers detained the three men in Hainan for posing a threat to national security.
Since 2015, Chinese authorities have detained at least five other Japanese citizens for suspected spying.
Chinese courts have begun hearings into separate cases involving four of those detained Japanese citizens. Under Chinese law, the maximum penalty for spying is death.
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 more than four years ago.
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.
In line with this trend, the Beijing municipal government last month began offering cash rewards of up to around $70,000 for information used to identify foreign spies.
During Xi's presidency, China has also tightened control over lawyers, journalists and civil society groups, and passed laws designed to fend off what the Communist Party sees as internal and external threats.