China said on Monday that it was detaining a Japanese national on suspicion of espionage, after Tokyo urged Beijing to release one of its citizens.

"Relevant Chinese authorities took criminal coercive measures this month against a Japanese citizen, in accordance with the law," foreign ministry spokesperson Mao Ning told a regular press briefing.

"This Japanese citizen is suspected of engaging in espionage activities, in violation of the criminal law and the anti-espionage law of the People's Republic of China," Mao said.

"China is a country under the rule of law. All foreign nationals in China must abide by Chinese laws, and offenders are prosecuted according to law," she added.

Japanese government spokesman Hirokazu Matsuno told reporters earlier on Monday that Tokyo's embassy in China had been informed "this month that a Japanese man in his fifties was detained in Beijing".

He gave no details on the man's identity, nor his alleged crime, or when he had been arrested.

"Ever since we learned about this case, the Japanese government has been strongly urging the immediate release of this Japanese national," Matsuno said.

He added that Tokyo was also seeking consular access to the man.

In October 2019, Chinese authorities detained a Japanese professor, reportedly on suspicion of spying. He was released and returned to Japan the following month.

And in March 2020, China's foreign ministry said it had arrested a Chinese man reportedly working as a university professor in Japan who they claimed had confessed to spying.