BEIJING, China (AFP) — Foreign spies are fitting turtles and fish with sensors to create underwater maps of China's coastline, Beijing warned Friday in apparent reference to its Western competitors.

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 defenses, 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.