Japan has confirmed that Chinese authorities have arrested a third Japanese man and detained another on suspicion of spying.

A Japanese government spokesman said at a press conference on Friday that two of its citizens were also arrested in September this year for espionage.

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Secretary Yoshihide Suga told reporters that a Japanese woman had been detained by the Chinese police in Shanghai since June and was formally arrested only last month. Suga, however, refused to elaborate on the details of the arrest and was mum on whether the woman was being held in Beijing for spying activities.

Reports indicate that a Japanese man has also been under criminal detention after being detained in Beijing since June. The Japanese media said the man has been accused of helping North Korean citizens in fleeing their country.

Suga said in all, four Japanese nationals are now in China's custody for allegedly committing spying activities on the mainland.

"Japan does not engage in spying activities in any country," Suga said.

President Xi Jinping has prioritized security laws upon assuming the presidency in 2013, tightening the already strict security regulations imposed by China.

As part of President Xi's security projects, he set up a new national security commission and gave a new name to  the national security law - the Counterespionage Law. The law has been in effect since 1993.

Four Japanese nationals were arrested by Chinese police in 2010 for alleged spying after the four men allegedly entered a Chinese military zone and took pictures without authorization.

The four men were later released after a month and flew back to their home country.

Experts say the series of arrests and detentions by China could strain the already tense ties between the two countries.

The two nations are locked in a bitter territorial dispute in the East China Sea as both have overlapping claims in several islands said to be rich in maritime resources.

TagsJapanese Secretary Yoshihide Suga, Chinese police, Japan's spying activities, Japanese media, four Japanese men in China's custody