China's Ministry of State Security says a US citizen who was sentenced to life in prison on espionage charges earlier this year had worked as a spy for a US intelligence agency for more than three decades.

The ministry released on Monday an account of the man, who holds permanent residency in Hong Kong and has a US passport. A court in the eastern Chinese province of Jiangsu gave him a life sentence in May. He was 78 years old at the time.

The ministry says the man signed a deal in 1989 to cooperate with and be paid by a US intelligence agency.

It says as a US spy, the man monitored Chinese nationals and overseas Chinese in the United States and reported to the intelligence agency.

The ministry says the man had been gathering intelligence in China as instructed since 2020, when the coronavirus was spreading around the world.

Hong Kong media say the man had been long engaged in friendly activities between the US and China.

The leadership of Chinese President Xi Jinping has been calling on citizens to join its fight against spying, saying counterespionage work requires the mobilization of all parts of society.

The ministry apparently announced details about the man because it aims to urge citizens to be on alert.