China’s seizure of a US navy submarine drone signals its willingness to push back harder against American surveillance in the South China Sea, as tensions mount between the two nations ahead of the presidential inauguration of Donald Trump.

The Pentagon said China had agreed to return the surveillance device, which was seized in international waters on Thursday. But the incident highlights concern about the direction of US-China ties amid uncertainty about the stance Mr Trump will adopt.

Mr Trump has been critical about China since launching his campaign, on everything from trade policy to its assertive moves in the South China Sea. After his victory, however, he sparked alarm in Beijing and among many experts in Washington by speaking to the Taiwanese president and hinting that he might abandon the “One China” policy that has guided US policy on China and Taiwan for four decades.

The president-elect cricised China over the drone incident, which he said was “unprecedented » before saying that the US should allow China to keep the device. “We should tell China that we don’t want the drone they stole back — let them keep it!” he tweeted.

While US-Sino relations usually enter a period of uncertainly at a time of administration change in Washington, analysts said that the apprehension surrounding the drone in disputed waters bucks the recent trend of improving ties between the US and Chinese navies.

“This goes against the grain of the professionalisation of [China’s] People’s Liberation Army Navy over the past few years, as it seemed to be embracing confidence-building measures with the US,” said Euan Graham, a security analyst at the Lowy Institute, a think-tank in Sydney. “This is a significant escalation with a navy ship interfering with another navy ship at close quarters.”

Ni Lexiong, a naval expert at the East China University, said the seizure was “a reminder to surrounding countries in the South China Sea area, as well as the US and Japan, to reconsider China’s stance on defending its territory”.

The Pentagon said the drone, which was being operated by the USNS Bowditch, a civilian-operated navy survey vessel, was taken “unlawfully” by a Chinese naval ship on Thursday, 50 nautical miles from Subic Bay in the Philippines. Such drones are typically used to collect oceanographic data such as water depth and salinity.

China’s defence ministry said that it had seized the drone “to prevent it endangering the navigation and crew” of the Chinese ship. But it also reiterated its opposition to US military reconnaissance operations in the waters around China.

Analysts said that while the information gleaned from underwater drones was not classified, it was vital to the operation of the submarines that both sides operate in the South China Sea.

Li Mingjiang, a China expert at the S. Rajaratnam School of International Studies in Singapore, said Beijing was willing to risk an escalation with Washington over the latest incident because it was worried about US plans to expand the use of naval drones in the South China Sea.

“PLA leaders have good reason to worry about this emerging US naval strategy against China in East Asia,” he said. “If this strategy becomes reality, it could be quite detrimental to China’s national security.”

Yue Gang, a retired PLA colonel, said that while the Bowditch might have been in international waters, it was “almost certain that the drone was conducting espionage activities” within the nine-dashed line, Beijing’s contested claim to much of the South China Sea.

He said that Beijing’s response was a “significant signal” of its displeasure with US policy, while also giving the navy time to analyse the drone to obtain “evidence of the espionage activities”.

Western analysts said that such a brazen act would undermine other nations’ confidence in China’s commitment to global norms.

“It’s illegal and it fuels concerns about whether China is really willing to abide by the international right of freedom of navigation,” said Ian Storey, a senior fellow at the Institute of Southeast Asian Studies in Singapore.

Security analysts also said that the seizure would prompt the US and others to rethink how they deploy civilian survey vessels and underwater drones.

“There will have to be a reappraisal of how unarmed US vessels do business in the South China Sea,” said Mr Graham of the Lowy Institute.

“They’re going to require an escort or armed personnel on board.”

Heiko Borchert, a security consultant, said that China’s actions highlight the risks of deploying drones in a “non-benign naval environment”.

“The incident shows that the tactical and strategic benefits of unmanned underwater vehicles can quickly turn into a strategic liability if other actors are not willing to back down on their own policy line,” he wrote on the website of the Center for International Maritime Security, a US think-tank.