A Chinese navy warship seized a U.S. underwater drone collecting unclassified data in international waters in the South China Sea, defense officials revealed Friday.

The drone was deployed by a U.S. oceanographic vessel operating in the region, reports Reuters. The incident occurred in Subic Bay Dec. 15, just as the USNS Bowditch was about to retrieve the unmanned underwater vehicle (UUV).

The drone was mapping the sea floor and collecting oceanographic data.

The Chinese warship, a Dalang III-class vessel (ASR-510), stole the UUV in full view of the American ship, which was operating about 100 miles from a nearby port.

The incident occurred at around noon Thursday (local time), Fox News reports.

The Bowditch is owned by the U.S. Navy but operated by Military Sealift Command. The ship “supports worldwide oceanography programs, including performing acoustical, biological, physical and geophysical surveys,” the website states.

The Bowditch was shadowed by a Chinese naval vessel, which dispatched a smaller craft to retrieve the drone. The American ship informed the Chinese crew that the vessel was U.S. property; however, they did not receive a response from the Chinese, CNN introduced.

The Chinese crew only responded as they were leaving.

The U.S. Department of State lodged a formal protest Friday and officially demanded the return of the drone, a Slocum Glider worth $150,000.

“The UUV 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,” Pentagon Press Secretary Peter Cook stated.

“This is an act of war against the United States,” Gordon Chang, a renowned China analyst told Fox News. He remarked that the theft is an act of war because U.S. military equipment was stolen.

The motive behind the blatant theft of U.S. equipment by the Chinese military is unknown; however, this incident follows a marked increase in tensions between the U.S. and China triggered by President-elect Donald Trump’s recent statements on Taiwan.

“China is clearly trying to test the United States in a way that is very hard for Washington to respond to,” Director of Defense Studies at the Center for the National Interest Harry Kazianis explained, “In stealing an underwater drone, Washington has very few viable options but to simply ask for it back, or escalate the situation in some manner, driving up tensions.”

“This was very likely a highly planned and escalatory move to show China will not take matters lightly when it comes to President-elect Trump’s phone call and comments on Taiwan, or Chinese actions overall,” he added.

China has warned multiple times in recent weeks that it may retaliate against the U.S. for its failure to respect China’s “core interests.”

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