BEIJING (Reuters) - China is investigating six Japanese citizens on suspicion of ‘illegal activities’, China’s foreign ministry said on Monday, after Japanese media reported they may have been suspected of spying.

Ministry spokeswoman Hua Chunying told a regular news briefing that Japanese consular authorities had been informed of the investigation.

“According to what I understand, the relevant Chinese department is, in accordance with the law, investigating six Japanese citizens on suspicion of engaging in illegal activities in China,” Hua said. She declined further comment.

Japan’s Kyodo news agency cited an unidentified Japanese government source as saying the six men had possibly been detained on suspicion of espionage.

Japanese Chief Cabinet Secretary Yoshihide Suga confirmed that six men had been detained but declined to comment on allegations of espionage.

“We were notified by China that three Japanese men each, six in total, had been detained in March by the Chinese authorities in Shandong Province and Hainan Province,” Suga told a regular briefing in Japan.

Both of the provinces have big Chinese military bases.

“We are providing them appropriate support through our diplomatic establishments abroad in light of protecting Japanese nationals,” Suga said.

Relations between China and Japan have been strained for decades by the legacy of Japan’s wartime aggression.

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.