China’s navy has seized a U.S. underwater survey drone in international waters in the South China Sea, prompting a formal diplomatic protest and a demand for its return Friday, in a confrontation likely to exacerbate already rising tensions.

Thursday’s seizure was the first of its kind in recent memory, and took place about 50 nautical miles (93 kilometers) from Subic Bay in the Philippines as the USNS Bowditch was about to retrieve the unmanned underwater vehicle (UUV).

The move coincided with a rise in saber-rattling from Chinese state media and its military establishment after U.S. President-elect Donald Trump criticized Beijing over the South China Sea and the nearly four-decade U.S. policy of viewing Taiwan as part of “one China.”

The Pentagon confirmed the incident and said the drone used commercially available technology that cost about $150,000. The Defense Department viewed the Chinese move as serious since it had effectively stolen U.S. military property.

“It is ours, and it is clearly marked as ours, and we would like it back. And we would like this not to happen again,” Pentagon spokesman Capt. Jeff Davis said.

Beijing had yet to release an official statement on the incident, but the state-run Global Times newspaper said China had seized “unidentified” equipment it found in the waters.

The paper cited a Chinese military source as saying Beijing believed the incident would be resolved “smoothly.”

A Chinese government-affiliated think tank on the mainland, meanwhile, said the incident highlighted how the two nations’ militaries had entered a new phase of underwater competition, the South China Morning Post reported Saturday.

“China wants to send out a signal that if you spy on us underwater and threaten our national security, we have measures to deal with it,” the paper quoted said Wu Shicun, president of the influential National Institute for South China Sea Studies, as saying. “On the South China Sea issue, we took in humiliations with a humble view in past years. I think this era has finished.”

The Defense Department said the Bowditch and the drone — an unclassified “ocean glider” system used around the world to gather military oceanographic data such as salinity, water temperature and sound speed — were conducting routine operations in accordance with international law when a Chinese naval Dalang-3 submarine rescue vessel launched a small boat and intercepted the UUV.

The Pentagon said the Bowditch made contact with the Chinese ship via bridge-to-bridge radio to request the return of the UUV. This contact was acknowledged but the request was ignored.

“As (the Chinese ship) went sailing off into distance, (it) said, ‘We are returning to normal operations,'” Davis said.

In a statement released later, Pentagon spokesman Peter Cook said that the drone was “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,” he said.

Regional observers called the move unprecedented.

“Although the drone was not conducting a military operation, disrupting and then stealing the naval platform — which has sovereign immune status — is one of the most brazen actions that the People’s Liberation Army Navy has taken against the U.S. Navy for a very long time,” said Ashley Townshend, a research fellow at the United States Studies Centre at the University of Sydney.

The seizure will add to growing concerns over China’s increasingly bellicose moves in the region.

It will also be a top issue for Trump, who campaigned on a pledge of extracting more favorable trade deals from China and has stoked anger in Beijing by taking a phone call from Taiwan’s president and slamming China for building what he called a “massive military complex” in the South China Sea.

Bonnie Glaser, director of the China Power Project at the Center for Strategic and International Studies in Washington, said Thursday’s seizure could be linked to Trump’s war of words with Beijing.

“It cannot be ruled out that this action was a deliberate signal to the incoming Trump administration, which the Chinese see as challenging their core interests on Taiwan,” Glaser said. “They have five weeks to persuade Trump to buy into the framework that has prevailed under (U.S. President Barack) Obama — cooperate where U.S. and Chinese interests overlap and manage/control differences where our interests diverge.”

Experts say Chinese naval and coast guard vessels typically operate within the bounds of guidelines that are dictated by officials in Beijing, meaning incidents such as this one may have been sanctioned by the leadership.

Others, however, believe this may always be the case.

“This, of course, does not mean that all actions are deliberate,” Townshend said. “Unsanctioned behavior by overly assertive ship operators or local military commands has occasionally taken place and could account for this latest provocation.”

Deliberate or not, experts said the move was unheard of in recent years as Washington and Beijing worked to prevent repeats of similar confrontations at sea and in the air while also building up a string of outposts in the South China Sea from which to project power.

Beijing claims most of the strategic waterway, through which more than $5 trillion in annual trade passes. Brunei, Malaysia, the Philippines, Taiwan and Vietnam all have rival claims.

It has reclaimed more than 1,280 hectares (3,200 acres) of land on seven features it occupies in the disputed Spratly chain. In July, the Permanent Court of Arbitration in The Hague rejected China’s expansive claims to much of the waters, a ruling Beijing criticized as “waste paper.”

The last major confrontation between the U.S. and China similar to Thursday’s happened in 2013, when a Chinese ship nearly collided with the USS Cowpens guided-missile cruiser after cutting in front of it at a distance of just 100 yards (91 meters).

“What we do know is this: China’s seizure of an American underwater drone is unprecedented,” Townshend said. “Over the past two years, Beijing has deliberately avoided risky or provocative tactical behavior in order to minimize the risk of a U.S.-China maritime crisis while it constructs a network of military outposts throughout the South China Sea. The incident on Thursday stands in stark contrast to China’s otherwise very cautious maritime behavior.”

The move also comes just on the heels of a report released Wednesday detailing a “significant” build-up by China of anti-aircraft and anti-missile systems on all seven of its man-made islands in the strategic waterway’s Spratly chain.

That revelation came despite a pledge last year by Chinese President Xi Jinping not to “militarize” the islands in the waters.

Beijing said it is “legitimate and lawful” for it to place defensive military equipment on islands where it has “indisputable sovereignty.”

“If someone was at the door of your home, cocky and swaggering, how could it be that you wouldn’t prepare a slingshot?” the Chinese Defense Ministry said Thursday in response to the report. The comments were likely in reference to U.S. “freedom of navigation” operations in the waters that have angered China. The last such operation came in October.

With Trump set to take office on Jan. 20, the charged atmosphere in the area is unlikely to abate soon.

“Against a background of rising tensions in the South China Sea and Trump’s increasingly hawkish comments on China policy, this incident will be a serious test for U.S.-China relations,” Townshend said.

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