Official says drone deployed by American oceanographic vessel in South China Sea was taken by Chinese navy on Thursday

The Chinese navy has seized an underwater drone in plain sight of the American sailors who had deployed it in international waters, in a seemingly brazen message to the incoming Trump administration.

According to a US defence official, the unmanned glider had come to the surface of the water in the South China Sea and was about to be retrieved by the USNS Bowditch, an oceanographic and surveillance ship, when a Chinese naval vessel that had been shadowing the Bowditch put a small boat in the water.

Chinese sailors in the small boat came alongside the drone and grabbed it despite the radioed protests from the Bowditch that it was US property in international waters. The incident happened about 100 miles north-west of the Philippines’ port of Subic Bay.

The US has issued a formal protest and demanded the return of the glider.

Peter Cook, the Pentagon press secretary, said the Bowditch made radio contact with the Chinese ship and asked for the glider to be returned. “The radio contact was acknowledged by the [Chinese] navy ship, but the request was ignored,” Cook said.

“The UUV [unmanned underwater vehicle] is a sovereign immune vessel of the United States. We call upon China to return our UUV immediately, and to comply with all of its obligations under international law.”

The aggressive Chinese gesture comes at a time of rising tensions between China and the US in the South China Sea, where Beijing has claimed ownership of a number of reefs and small islands – which it is in the process of militarising – while the US navy has been conducting patrols nearby to assert freedom of navigation in the sea lanes.

The tension has spiked since Donald Trump was elected in November. The US president-elect quickly broke a 37-year protocol by taking a call from the president of Taiwan, and openly questioned Washington’s longstanding “one China” policy that does not recognise Taiwan as a separate state. Beijing has signalled it would respond dramatically if Trump implements a break in policy once he takes office on 20 January. In recent days, China has conducted bomber patrols close to Taiwan in a flexing of its military muscle.

The seizure of the drone is also a reflection of the struggle occurring under the surface of the South China Sea. As China develops a strategic submarine fleet, with the potential to carry nuclear missiles out into the Pacific Ocean, the US has built up a monitoring network designed to spot Chinese submarines as they leave their bases. Drones are key to the network, and there is a race under way between major naval powers to develop drones that can work together in swarms and “see” long distances through the water. Underwater gliders are drones that can stay underwater on the lookout for submarines for long periods of time.

“This looks like signalling from the Chinese in response to Trump’s Taiwan call,” said Bonnie Glaser, the director of the China Power Project at the Centre for Strategic and International Studies. “It is hard to believe this is the action of an independent commander. The Chinese now have much better control over the military, particularly the navy. It is in China’s interest to send signals before Trump is inaugurated, so that he gets the message and be more restrained once he is office.”

Sebastian Brixey-Williams of the British American Security Information Council said: “Nuclear states are increasing anxious about unmanned underwater vehicles (UUVs, or underwater drones) autonomously tracking their nuclear ballistic missile submarines (SSBNs), making them vulnerable to antisubmarine warfare. This is an issue for China in particular, whose SSBN fleet is small and noisy. Though the USNS Bowditch is an oceanographic ship and may sound harmless, the kinds of data it is collecting will make Chinese submarines easier to find over time.

“China therefore accomplishes a number of things by seizing a US underwater drone,” Brixey-Williams said. “It allows Chinese scientists to better understand the US’s offensive technical capabilities in this area, and potentially allows them to reverse-engineer them, bringing gains in both the commercial and military spheres.”

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Glaser pointed out that the Chinese have frequently tested the US when there is a new administration. In the early months of the George W Bush administration, in 2001, the Bowditch was involved in a close encounter with a Chinese frigate which turned on its gun control radar and forced it to retreat. A week later there was a collision between a US spy plane and Chinese warplane off China’s Hainan island.

At about the same point in the early Obama administration, in March 2009, a number of Chinese navy ships harassed another US oceanographic vessel, the USNS Impeccable, coming as close as 50ft away, trying to snag its acoustic equipment with hooks, waving flags and demanding the Impeccable leave the area.