The torpedo-shaped drone, which is about three metres long and weighs 200kg (441lbs), is equipped with artificial intelligence technology and a wide range of sensors to detect temperature, salinity, current, trace chemicals, underwater visibility and biological activity.

It can dive 2,000 metres below the surface and cruise at speeds of up to 1.2 metres per second.

“Sea-Whale 2000’s development is to meet the long-term deep-sea mobile survey needs in the South China Sea,” said an announcement by the programme team, led by Dr Huang Yan of the Chinese Academy of Sciences’ robotics institute in the northeastern Liaoning province.

The drone is able to run “a long-endurance mission for weeks with one launch and can accomplish multiple working mode missions”, they said.

They declined to elaborate on why China is building the drone for the South China Sea.

But China is gearing up for a race in developing unmanned submarines and is among the first to have put the technology into service.

To extend the drone’s range, Huang and colleagues came up with an unusual hybrid structure that would allow it to run in different modes to reduce energy consumption. It can cruise like a conventional submarine, glide up and down, or ride the tide like a drifting bottle.

The drone uses a low-power central processing unit from STMicroelectronics, a semiconductor company based in Europe, and communicates using a global satellite network run by Iridium Communications, an American firm.

China is not the only country concerned with its capabilities in the South China Sea. Over the decades, the United States and its allies have established extensive surveillance networks in the region, with military and scientific research vessels conducting regular patrols in disputed parts of the sea to gather intelligence.

About 10 years ago, Beijing launched an ocean monitoring network intended to be the largest in the South China Sea, according to researchers involved in it.

The People’s Liberation Army Navy and many civilian research institutes have deployed a large number of buoys and monitoring stations moored on the sea floor. Their density in the area is now “far higher” than similar facilities run by the US and other countries, the Chinese researchers said.

But these stationary facilities cannot cover the entire sea and their maintenance is costly. Some have sustained accidental or seemingly intentional damage. Chinese scientists told the South China Morning Post the long-range drone would help their work.

Professor Liu Xiaoshou, a marine life scientist at the Ocean University of China, said the biological sensors on Sea-Whale 2000 could collect data on ecological matters such as algae bloom.

Together with information gathered by other sensors, scientists could reconstruct the evolution of biological systems in greater scale.

“I hope I can use the technology in my study,” Liu said.

Plenty of data collection work is still carried out by humans on research voyages, which are expensive, time-consuming and affected by the weather.

But Liu said large-scale deployment of the drones may not happen immediately because the new technology would need more time to prove its worth.

Data collected by the drone would need to be compared carefully with that collected by other methods to determine its accuracy and quality.