Is Tokyo dispatching operatives to spy on its neighbor and rival, or are innocent civilians becoming caught up in Sino-Japanese geo-political tensions? Julian Ryall reports from Tokyo.
After four years in which bilateral tensions have run high over the issues of territory and differences in the interpretation of their shared history, China and Japan have appeared in recent months to be rebuilding bridges. High-level delegations of Japanese businessmen have visited Beijing to underline the economic importance of the relationship, discussions are taking place on a summit between Prime Minister Shinzo Abe and Chinese Leader Xi Jinping, while State Councilor Yang Jiechi is presently in Tokyo to help smooth over differences between the two nations.
Yet the arrest of four Japanese nationals by Chinese security officials on charges of spying on behalf of the Japanese government - a claim that Tokyo strongly denies - threatens to undermine that detente.
The Chinese government announced on October 11 that it had arrested a woman in her 50s in the city of Shanghai in June. A man in his 60s has also been detained in Beijing on similar espionage charges.
Two earlier arrests
China and Japan have differences over the issues of territory and the interpretation of their shared history
On September 30, Beijing announced it had detained two Japanese citizens in separate cases in Liaoning and Zhejiang provinces. The Japanese government has confirmed that both men, aged 51 and 55, work in the private sector, but Chief Cabinet Secretary Yoshihide Suga responded unequivocally when asked whether the two men were spying on behalf of Tokyo.
"Our country has never done such a thing," he said at a press conference, adding that China should release the detainees immediately.
That denial was dented by a report by Japan's Kyodo News citing a Japanese government source that the two men - one of whom is a defector from North Korea who was arrested near the China-North Korea border - were, in fact, collecting information about Chinese military activities for Japan's Public Security Agency.
Jun Okumura, a visiting scholar at the Meiji Institute for Global Affairs, says he finds it hard to believe China's allegations.
"From the media reports, I understand that one of the men arrested earlier was taking photos of Chinese military aircraft and airfields, but that sounds more like curiosity than spying to me," he told DW.
"What sort of intelligence could be gained by standing outside an airfield fence?" he asked. "There are far better ways of collecting useful information - satellite images, electronic eavesdropping, signals intelligence - than a camera through a fence."
Equally, the North Korean defector detained close to the border may well have been involved in assisting other people fleeing the regime of Kim Jong-un to reach safety beyond the borders of China, which typically sends defectors back to Pyongyang.
Security has been tightened in China in recent months, with Beijing enacting new espionage legislation in November. Analysts say the new law applies to the theft of national secrets and bribing public officials - but which also includes a catch-all phrase that cites "any other spying activity" - is designed to make it easier to monitor the movements and activities of foreign nationals inside China.
Okumura believes that the timing of the arrests is curious, given that Beijing would be unlikely to "want to throw a spanner in the engine of gradually improving Sino-Japanese ties," although he added that a conspiracy theorist might surmise that factions in Beijing opposed to the policies of the liberal regime of President Xi - and particularly his reaching out to Tokyo - may have engineered the arrests.
An alternative suggestion is that it may simply be a tit-for-tat response to previous allegations by Japan concerning Chinese agents operating here. In 2012, Japanese police alleged that a former diplomat stationed at the Chinese Embassy in Tokyo was conducing in espionage, while earlier this year another diplomat was allegedly involved in efforts to purchase the headquarters of the North Korean government in Tokyo, effectively the regime's embassy in Japan.
China's assertiveness in South China Sea has drawn Japan and other smaller nations in the region closer
Tokyo has for many years also been wary of industrial espionage carried out to steal Japan's cutting-edge technological secrets by Chinese agents, often allegedly using "honey traps" designed to blackmail high-ranking executives and engineers of leading Japanese companies.
In a statement to DW, the foreign ministry in Tokyo would only confirm that it is "aware" of the detention of two Japanese in China.
"We are still working on getting all the facts," a spokesman for the ministry said. He would not comment on the two most recent arrests.
The arrests have triggered anger in Japan, with the Yomiuri newspaper condemning Beijing's actions in a recent editorial.
Describing the detentions as an effort by the Chinese government "to defend its one-party dictatorship at all costs by bolstering its domestic control even further," the newspaper added that the arrests may be intended to "rattle" the Japanese government. It also described China as an "aberrant superpower."
The Chinese embassy in Tokyo could not be contacted for a comment.