SHANGHAI (Reuters) - China is investigating a Japanese citizen on suspicion of endangering national security, China's foreign ministry told Reuters on Saturday in a faxed response to questions.

The fax confirmed a report from Japan's Kyodo news agency on Friday, and added that the Japanese embassy had been informed.

The man, in his late 50s, was scheduled to stay in Beijing for five days through July 15, but did not return to Japan and has not answered his mobile phone, Kyodo cited Japanese government sources and others as saying.

The Japanese government said on Thursday that a Japanese man was detained in China, but did not say on what charges. When asked whether he was suspected of spying, Chief Cabinet Secretary Yoshihide Suga said Japan did not engage in espionage against any country.

Relations between the countries are strained by a territorial dispute and the legacy of Japan's wartime aggression.

Chinese President Xi Jinping has overseen a tightening of already strict security measures, including setting up a new national security commission and renaming the national security law, which took effect in 1993, the Counterespionage Law.

At least two Japanese citizens were arrested on suspicion of espionage last year.

In 2010, four Japanese nationals were temporarily detained in China on suspicion of entering a military zone and taking photographs without permission. The detentions came at a time of escalating tensions between Tokyo and Beijing over a group of East China Sea islets that both countries claim.

(Updates dateline, reference to previous request for comment.)

(Reporting by Kiyoshi Takenaka in TOKYO, Ben Blanchard in BEIJING and Pete Sweeney in SHANGHAI; Editing by Robin Pomeroy and Kim Coghill)