The USNS Bowditch, an oceanographic survey vessel with a mostly civilian crew, was in the process of recovering two unmanned ocean gliders, which are used to collect information about water conditions that can help U.S. vessels operate. A Chinese ship, a Dalang-III class submarine rescue vessel, approached the area, coming within about 500 yards of the Bowditch before dropping a small boat in the water. It seized one of the gliders and brought it aboard, Davis said.

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The Bowditch contacted the Chinese ship and asked for the glider to be returned. Officials aboard the Chinese ship acknowledged the radio communication, Davis said, but said they were returning to normal operations. The ship then left the area.

“We would like it back and we would like this not to happen again,” Davis said, referring to the underwater drone. The incident occurred around 1:45 p.m. local time, the Navy said.

A defense official, speaking on condition of anonymity because he was not authorized to discuss the matter on the record, said the United States has made an official diplomatic communication to the Chinese government, “demanding the return of our stuff.”

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There was no immediate response from Beijing, the official said.

The incident comes amid mounting tensions over Beijing’s territorial claims in the South China Sea, which have alarmed U.S. allies in the region and prompted the U.S. military to conduct “freedom of navigation” operations intended as a show of force. This week, a U.S. think tank reported that China has placed antiaircraft weapons on artificial islands, threatening to intensify the debate over disputed areas.

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Davis said ships such as the Bowditch routinely conduct operations in the South China Sea. The Navy said the drones are operated by the Naval Oceanographic Office, which states in its promotional materials that it maintains the largest glider fleet in the world, with more than 130 “littoral battlespace sensing” crafts.

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The gliders are piloted by civilian employees of the oceanographic office from Stennis Space Center in Mississippi through the use of encrypted satellite communications. They typically travel just a few miles per hour and are tracked by oceanographic vessels such as the USNS Bowditch. The data the drones collect is unclassified.

Wu Shicun, president of China’s National Institute for South China Sea Studies, said the unmanned vehicle posed “a threat to China’s national security” because it could be used to “spy” on China’s land reclamation efforts or collect information about Chinese submarine routes.

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“Why are [the underwater vehicles] there? I think the U.S. knows it very well,” he said.

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Sen. John McCain, (R.-Ariz.), chairman of the Senate Armed Services Committee, said Friday afternoon that the seizure is a “flagrant violation” of the laws of the seas.

“China had no right to seize this vehicle,” McCain said. “And the United States must not stand for such outrageous conduct.”

McCain said the incident fits a pattern of increasingly destabilizing Chinese behavior.

“This behavior will continue until it is met with a strong and determined U.S. response, which until now the Obama administration has failed to provide,” McCain said. “Freedom of the seas and the principles of the rules-based order are not self-enforcing. American leadership is required in their defense.”

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Chinese analysts, however, downplayed the incident. “It won’t have too much impact on the Sino-U.S. relationship,” said Zhu Feng, dean of the School of International Relations at Nanjing University. “China and the U.S. have been through conflicts far more severe.”

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It’s not clear how President-elect Donald Trump, who has been the subject of Chinese government ire since his recent outreach to leadership of Taiwan, will approach those maritime disputes. But he has criticized Beijing for “building a massive fortress in the middle of the South China Sea,” in addition to his complaints about Chinese monetary and trade policies.

Patrick Cronin, an expert on Asia-Pacific security at the Center for a New American Security, called the seizure “a brazen, calculated act of coercive diplomacy” and said it was intended to send a message to Trump ahead of his inauguration.

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“Rather than wait several weeks, Beijing is advancing a provocative action offshore from a U.S. ally that had recently kowtowed to China,” Cronin said, referring to the incident’s location near the Philippines. The signal China wishes to send is unambiguous, he said: “‘If you challenge our sovereignty we will challenge yours.’ The U.S. response needs to be equally clear: if anyone messes with with our Navy the response will not be limited to words.”

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Both sides are merely signalling, said Qiao Liang, a military commentator and major general in the Chinese air force. “You called Tsai Ing-wen, sending a signal; we capture the UUV in South China Sea, our own exclusive economic zone,” he said. “It’s certainly a signal to the U.S.”

But, he added, “there will not be any escalation before Trump takes office.”

Luna Lin contributed from Beijing.