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 employees of NC Geophysical Survey Co., a Japanese company based in Funabashi, a city located about 20 kilometers east of Tokyo.

The company said it believes the four did not do any wrongdoing and it was told by the Japanese Foreign Ministry that they are in good shape.

China's Foreign Ministry spokeswoman Hua Chunying, speaking at a regular press briefing, said they are under investigation in accordance with China's law.

Three of the men were detained in the eastern province of Shandong and the other three in the southern island province of Hainan, according to the source.

It is unclear why they are being held, but authorities may suspect them of spying due to the presence of major Chinese naval ports and other facilities in those two provinces.

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. The three men, who are in their 20s to 50s, arrived there in late March for hot spring development-related work.

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.