Chinese authorities have formally arrested a Japanese businessman detained earlier this year on suspicion of espionage, the Japanese government said Thursday, despite its repeated calls to release him.

Japanese Chief Cabinet Secretary Hirokazu Matsuno told a press conference that the government confirmed the arrest in mid-October of the man in his 50s, saying Tokyo will continue to urge Beijing to release him as soon as possible by working "at various levels."

The man is an employee of Japanese drugmaker Astellas Pharma Inc. and has been detained by Chinese authorities in Beijing since March. China told Japan last month the man had been placed in criminal detention -- a step leading to official arrest.

Chinese Foreign Ministry spokeswoman Mao Ning said at a press conference, "We will handle the case in accordance with law and protect the lawful rights and interests of the person concerned."

An Astellas Pharma official told Kyodo News the company will "continue to deal appropriately with the situation through the (Japanese) Foreign Ministry."

The arrest comes as relations between the two Asian neighbors remain strained following the start in late August of Japan's discharge of treated radioactive water from the crippled Fukushima nuclear power plant into the sea.

The Japanese citizen, who formerly served as a senior official of the Japanese Chamber of Commerce and Industry in China, was detained just before his scheduled return to Japan. The specific details of how he may have violated the counterespionage law and criminal code in China remain unknown.

On July 1, a revised counterespionage law took effect in China, broadening the scope of what constitutes spying activities. As the definition of national security remains unclear, the legislation has raised fears among expatriate and foreign business communities.

The news of the Astellas employee's formal arrest disappointed some Japanese participants at a two-day forum on bilateral relations held in Beijing through Friday.

Yasushi Kudo, head of Japanese nonprofit think tank Genron NPO, urged China to explain the reason for the arrest, saying fears are spreading among Japanese nationals living in China. The entity is a co-organizer of the event attended by experts of the two countries.

A Japanese researcher who took part in the forum expected that the number of visitors from Japan to China will further drop due to the incident, while a senior Japanese banker said the amended counterespionage law has had "a major negative impact" on the business sector's efforts to deepen exchanges with China.

Since China's counterespionage law first came into force in November 2014, 17 Japanese citizens including the Astellas employee have been detained for alleged involvement in spying activities. Five are still being held, according to the Japanese government.

Related coverage:

China to soon decide whether to arrest Japanese national over spying

China steps up scrutiny of Japanese after Fukushima water decision

Japan business leaders to make 1st China visit after COVID in January