Beijing has stepped up espionage warnings in recent years as relations with the United States and other Western nations have worsened.

Its Ministry of State Security (MSS) said Thursday that foreign spooks "have been increasingly active in infiltrating and stealing secrets from China" and were targeting public officials.

"Individual officials have caused the damaging effects of leaking secrets due to a lack of belief, a weakened sense of discipline and a loosened awareness of rules... harming national security and interests," the MSS said.

It described a case in which a provincial government employee surnamed Li fell into a "meticulously designed honeytrap" while travelling overseas for work.

"Unable to resist the seductive beauty of the foreign intelligence agent," Li was then blackmailed with "intimate photos" and forced to hand over official documents once back in China.

He was sentenced to five years in prison for espionage, according to the statement.

Another case involved a municipal cadre called Hou, who secretly photographed confidential documents and sold them to foreign spy agencies to recover lost savings from a gambling addiction.

Hou was later held "criminally responsible", the ministry said, without specifying his punishment.

The ministry also detailed a third case in which a high-flying young official lost his job after sharing confidential information with a relative who photographed and sent it to overseas spy agencies.

"Leaks are often hidden in the small details of our work lives," the ministry said.

"If our ideals and beliefs are not strong... they may eventually plummet into the criminal abyss designed by foreign spy agencies."

China's spy agency did not name the countries alleged to be behind the plots.

Asked which countries were involved by AFP at a regular briefing on Thursday, foreign ministry spokeswoman Mao Ning said she was "not familiar with this issue".

Beijing and Washington have long traded accusations of espionage, punishable by death in China.

Last month, Beijing accused the US Central Intelligence Agency of an "absurd" attempt to recruit Chinese citizens via "amateurish" videos posted on social media.

In April, Chinese security officials said they had implicated three US "secret agents" in cyberattacks during February's Asian Winter Games in the northeastern city of Harbin.

The state security ministry also said in March it had sentenced to death a former engineer for leaking state secrets to a foreign power.