Chief Cabinet Secretary Yoshihide Suga said Friday a third Japanese citizen has been arrested in China, on the heels of the arrests of two others earlier this year.

Suga told a regular news conference that the woman, who was taken into custody in Shanghai in June, was formally arrested last month. He gave no further details, including whether she is suspected of spying.

In September, China said it had arrested two Japanese for spying and a Chinese Embassy official in Tokyo later confirmed that two others were being held.

Suga said the fourth person was a man, said by Japanese media to have been helping people flee North Korea, who was taken into custody in June and is now under criminal detention.

Suga reiterated that Japan does not engage in spying in any country but declined further comment on the specific cases.

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

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.

Chinese ties with Japan have long been troubled by a territorial dispute and what Beijing sees as Tokyo's failure to properly atone for wartime atrocities, as well as regional rivalry and mutual military suspicion.

However, relations have improved since Japanese Prime Minister Shinzo Abe and Xi met in November last year.