“China steals United States Navy research drone in international waters - rips it out of water and takes it to China in unpresidented act,” Trump wrote on Twitter. A short time later, he deleted the tweet and reposted it with "unprecedented" spelled correctly.

China steals United States Navy research drone in international waters - rips it out of water and takes it to China in unprecedented act. — Donald J. Trump (@realDonaldTrump) December 17, 2016

The Pentagon on Friday demanded that China return the unmanned underwater vehicle it took in the South China Sea.

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The department said in a statement it is using "appropriate government-to-government channels" to call upon China to return the drone "immediately."

The incident occurred Thursday around noon local time in international waters off the coast of the Philippines, according to a defense official.

The USNS Bowditch, an oceanographic survey ship, was preparing to retrieve its unmanned drone out of the water, as part of their typical mission to collect data on the ocean and weather patterns, the official said. The drone had surfaced and sent out a signal as to its location per normal operations.

A Chinese ship that had been shadowing the Bowditch then dropped its own small boat in the water and swooped in to grab the drone, the official said.

The Bowditch crew then called over via radio to the Chinese ship to ask for their equipment back. The Chinese crew confirmed receipt of the message, but began sailing away, leaving with the drone.

Around noon local time on Friday, the U.S. State Department filed an official demarche with China. The official said the matter is now in the State Department's hands.

China's seizure of the drone is likely a signal to President-elect Donald Trump that it won't take his phone call with Taiwan lightly, an expert said on Friday.

"Knowing Chinese military officials for many years and how orders are communicated from the highest power centers in Beijing down to commanders on the ground or water, 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," said Harry Kazianis, director of defense studies at the Center for the National Interest.

Trump took a phone call from Taiwanese President Tsai Ing-wen congratulating him on his presidential win, breaking decades of diplomatic protocol. No U.S. president or president-elect had spoken with a leader of Taiwan since the U.S. normalized relations with China in 1979. China considers Taiwan a breakaway province, not a separate country.

Both Trump and Tsai have played down the significance of the call, but it triggered a formal complaint from China to the U.S.

- Updated at 9:01 a.m.