China has formally arrested a Japanese businessman who was detained in March on suspicion of espionage, the Japanese government said on Thursday, calling on Beijing to release its national to avoid the issue becoming another thorn in relations.

“We have also strongly urged China for an immediate release at various levels and we will continue to do so,” Chief Cabinet Secretary Hirokazu Matsuno said, adding that the man, aged in his 50s, had been formally arrested in mid-October.

Kyodo news agency identified the man being held as an employee of drugmaker Astellas Pharma, though the foreign ministry has not confirmed that and an Astellas spokesman said the company was still gathering information via the ministry.

Under China’s judicial system, the formal arrest of a suspect triggers a more intensive investigation by prosecutors, before pressing charges.

Japanese media have reported that the individual detained has worked in China for 20 years, previously serving as a senior official of the Japanese Chamber Commerce and Industry in China.

The arrest could send shudders through foreign firms operating in China, where the government recently rolled out broader anti-espionage legislation that some countries fear could be used to punish regular business activities.

When asked about the arrest, China’s foreign ministry spokesperson said such cases are handled according to law, and individual’s rights and interests are protected.

As part of President Xi Jinping’s increased focus on national security, Beijing implemented a new anti-espionage law in July in a move to further tighten restrictions on foreigners.

The arrest could further chill Japan’s relations with China, which is both its rival and closest trading partner - China is Japan’s biggest source of imports and its biggest export market.

Japan recently suffered a backlash from China over the release of treated radioactive water from the Fukushima nuclear plant into the ocean.

Japanese companies have spent years building extensive supply chains and manufacturing facilities in the world’s second-largest economy, which is also a key market for Japanese manufacturers, including automakers.

Imports from China totalled $198 billion last year, while exports to China were $145 billion, according to IMF trade data.