TOKYO (Reuters) - Four of six Japanese citizens detained in China since March on suspicion of “illegal activities” have been released, Japan’s top government spokesman said on Thursday.

“Four out of six Japanese nationals detained in Shandong Province and Hainan Province in March have returned to Japan,” Chief Cabinet Secretary Yoshihide Suga told a news conference.

China’s foreign ministry said the six Japanese citizens were being held on suspicion of “illegal activities”. Both of the provinces have big Chinese military bases.

NC Geophysical Survey Co., in Funabashi city, near Tokyo, confirmed that three of the four men returned to Japan were its employees.

The company said it dispatched four employees to China in March for geological research after receiving orders from two Chinese hot spring developers. Its fourth employee is still being held in China.

Suga declined to comment on why four had been released.

“The government’s top responsibility is to protect Japanese nationals. We continue to provide support for those who are still detained,” said Suga.

A maritime territorial dispute over small islands in the East China Sea has in recent years added to the suspicion between the two sides.

In 2010, four Japanese nationals were detained in China on suspicion of entering a military zone and taking photographs without permission.

At least two Japanese citizens were arrested on suspicion of espionage in 2015. Last year, China said it was investigating a Japanese citizen on suspicion of endangering national security.