Beijing and Tokyo appear to have waited until two of China's key diplomatic and domestic events — the 70th anniversary of Japan's defeat in World War II and President Xi Jinping's visit to the United States — were over before revealing the detention of two Japanese nationals in China for alleged espionage, a Japanese expert on China said Thursday.

On Wednesday, China and Japan announced that two Japanese men were being held on charges of espionage by authorities in Zhejiang and Liaoning provinces, respectively, since May.

"Since there was a possibility that Prime Minister (Shinzo) Abe would attend a ceremony for the 70th anniversary, both sides probably wanted to withhold the information until it was over so that it wouldn't stir up public sentiment," said Satoshi Tomisaka, professor at Takushoku University's Institute of World Studies.

Abe, however, did not visit Beijing, after many Western leaders including U.S. President Barack Obama turned down invitations to attend a massive military parade to mark the event.

Abe also stayed in Japan to avoid any complication that might have stemmed from a visit to China while his security bills were being deliberated and passed by the Diet.

Xi's seven-day visit to the U.S. through Monday involved a bilateral summit with Obama and a speech at the U.N. General Assembly.

Aside from the timing of these key diplomatic events, Tokyo may also have withheld the information out of concerns for the safety of its two detained citizens.

"We don't want it to affect future negotiations with China," said a government source who asked to remain anonymous, referring to Tokyo's reluctance to reveal more information, Kyodo News reported.

One of the two men reportedly took photos of a military facility in Zhejiang province, while the other was seized near Dandong in Liaoning province, which shares a border with North Korea.

Bonji Ohara, a researcher on Chinese affairs at the think tank Tokyo Foundation, said the fact that Beijing had not publicly used the incident for the past four months, either as a political tool to curry favor with Tokyo or as propaganda to stir up anti-Japan sentiment, may mean Chinese authorities actually believe the two Japanese were engaged in espionage.

"If that is the case, the situation may be more serious," said Ohara, a former Maritime Self-Defense Force officer who had been posted in Beijing. "But now that it is made public, China may start to use it politically."

To ease concerns from the U.S. and other nations against China's military buildup, China has avoided being provocative this year, Ohara said.

"Worsening ties with Japan would only prompt the U.S. to criticize China, which is the last thing it would want at this point," he added, suggesting that it wouldn't make sense for China to fabricate an espionage case against Japan.

On Wednesday, Chief Cabinet Secretary Yoshihide Suga denied the possibility that Tokyo deployed the two men to spy on China.

Tomisaka of Takushoku University said the key was whether they will be indicted or not.

"The Japanese government may be able to negotiate with China (to have them released) if it is before their indictment. But once they are indicted" it would be hard to intervene, Tomisaka said. "China takes a harsh stance on issues like this, so if they are found guilty, the sentences may seem strict from the eyes of Japanese."

Under Xi's administration, China has been taking a harsh stance on espionage. Last November, China enacted a counter-espionage law aimed at tightening state security. The maximum penalty for spying is death.

In 2010, four Japanese nationals working for construction firm Fujita Corp. were temporarily detained in China on suspicion of entering a military zone in Hebei province and taking photographs without permission.

Of the four detained on Sept. 20 of that year, three were released 10 days later and the fourth just under three weeks after their arrests. In the 2010 case, China immediately disclosed the incident and criticized Japan.

In 2013, Zhu Jianrong, professor at Toyo Gakuen University in Tokyo, went missing after he visited Shanghai. China later effectively admitted that authorities were interrogating the professor to see if his research was in violation of Chinese law. He was released after six months.