US officials demand return of the drone they say was testing salinity and temperature in international waters.

A Chinese Navy warship has seized an underwater drone deployed by an American oceanographic vessel in international waters in the South China Sea, triggering a formal diplomatic protest from the US and a demand for its return, a US defence official said.

The “naval glider” was used to test water salinity and temperatures to help in the mapping of underwater channels, the official said, before adding “it was taken” by China in international waters about 50 miles off Subic Bay.

The incident took place on Thursday just as the USNS Bowditch, an oceanographic survey ship, was about to retrieve the unmanned, underwater vehicle, the official said.

Al Jazeera’s Kimberly Halkett, reporting from Washington DC, said the Chinese vessel put a smaller boat in the water and picked up one of the two drones just as the American vessel prepared to retrieve them.

“The Americans apparently sent a message to their Chinese counterparts, asking for the drone to be returned but the radio transmission was reportedly ignored,” she said.

“It’s not unusual for Chinese ships to be following American vessels under the assumption they are spying. Now the Americans are saying they are going through diplomatic channels to try and retrieve the drone.”

The Philippines, Brunei, Malaysia and Vietnam have competing claims in the South China Sea, which is laced with the world’s most heavily travelled international trade routes.

The vessel’s seizure comes amid rising tensions in the South China Sea, where China has moved to fortify its claims to the region by building out tiny reefs into artificial islands.

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New satellite imagery made public on Wednesday by a US-based think-tank showed that China apparently has installed what appeared to be large anti-aircraft guns and close-in weapons systems on seven islets in the Spratly chain.

The US military has conducted several “freedom of navigation” operations in which ships and planes have passed close to the sites Beijing claims.

Such missions have drawn howls of fury from China, which accuses Washington of provocation and increasing the risk of a military mishap.

Adding to the tension, Beijing is facing a new US president in Donald Trump, who has questioned long-standing US policy on Taiwan, called Beijing a currency manipulator and threatened Chinese imports with punitive tariffs.