In a social media post ominously titled "Under the deep blue, undercurrents are surging", the Ministry of State Security said international spy agencies are using "new types of espionage equipment" to steal sensitive marine data.

"Relatively large marine animals with sensors attached have been discovered in certain waters of China," the ministry said, in a section titled "spy turtles, spy fish".

The clandestine creatures were found "swimming in a specific area, collecting sensitive data about the marine environment such as water temperature, salinity and ocean current, transmitting it overseas via satellite", it said.

Foreign groups also used solar-powered wave gliders, buoys with high-precision sensors, and devices loaded onto cargo ships capable of capturing "port dynamics" in real time, it added, without naming a particular agency.

The data collected would be used to create "underwater maps" that can "identify weak points in China's coastal defences, posing a serious threat to China's national security", according to the ministry.

The ministry urged proper security checks on equipment received from abroad, and called on fishers to report any fishy-looking buoys or devices found at sea.

Beijing and Western governments have long traded accusations of espionage.

Last year Beijing warned government workers to remain vigilant of "honeytrap" schemes, after a public servant was lured by the "seductive beauty" of a foreign agent.

This month, the Five Eyes alliance of Western security agencies said Chinese spies were posing as job recruiters online to seek sensitive information.