Two more Japanese nationals have been detained in China on suspicion of spying, bringing the number of Japanese held by Chinese authorities for espionage to four, news reports said Sunday.

A Japanese woman in her 50s has been held in Shanghai since June for her alleged involvement in spying on the country, the Mainichi Shimbun daily said.

The woman, who runs a Japanese language school in Tokyo, had visited China frequently, Kyodo News said, adding that the purpose of her visits was unknown.

Separately, another Japanese national in his 60s has been detained in Beijing on similar charges, the Mainichi said.

China said in late September it had arrested two Japanese citizens for suspected spying, a move likely to strain already tense ties between Asia's two largest economies.

The detentions -- the first since 2010 involving Japanese on such charges -- come as relations remain clouded by disputes over islands and Tokyo's wartime history, despite close trade links.