A Japanese woman was detained in China in June on suspicion of spying, the fourth Japanese citizen to be held by Chinese authorities for the same reason, according to Japanese media on Sunday.

The woman in her 50s works in a Japanese language school in Tokyo, but the purpose of her visit to China is not known. The visit is one of several she had made to China, the Japan Times reported. She was arrested in Shanghai, but the charges against her have not been revealed.

The Asahi Shimbun reported that the detained woman is a native Chinese but had obtained Japanese nationality. At press time the Chinese foreign ministry had not yet issued a response.

Chinese authorities confirmed on September 30 that two Japanese on suspicion of acting as spies were arrested in China. The two men were believed to have been collecting information at the request of Japan's Public Security Intelligence Agency. One of them was being held in Northeast China's Liaoning Province, near the border with North Korea, according to Japanese media.

The other man was detained near a military facility in East China's Zhejiang Province. According to the Japan Times, another Japanese man in his 60s was detained in Beijing in June.

Japan's Chief Cabinet Secretary Yoshihide Suga denied that the Japanese government had sent spies to China in a press conference on September 30, saying "we have never done [such a thing]," the Asahi Shimbun reported.

Li Wei, an anti-terrorism expert at the China Institute of Contemporary International Relations, told the Global Times that Japan's dispatching spies to China is due to strained relations between the two countries.

Normally, the two countries would get to know each other through official meetings and other communications. However, with relations souring, Japan has resorted to sending spies to acquire information to serve its diplomatic and domestic policies, Li said.

Japan's policy with China is the other reason it sent spies to China, Liu Jiangyong, a professor with the Institute of International Studies at Tsinghua University, told the Global Times. The country has seen China as an imaginary enemy since 2012, and it holds a strong defensive mentality toward China, Liu said.

Li said military information is what Japan wants most, as they are eager to know China's military development and strategic deployment.

In recent years, Japan has also been paying increased attention to unconventional fields, collecting economic and environmental information, Li said.

Observers believe that the incident will further worsen the already strained Sino-Japanese ties.