The drone was taken on Thursday, the first seizure of its kind in recent memory. It prompted the Pentagon to go public with a diplomatic complaint, and drew criticism from U.S. President-elect Donald Trump, who has vowed to take a more aggressive approach in dealing with Beijing.

The drone was collecting data about the salinity, temperature and clarity of the water about 50 nautical miles northwest of Subic Bay, off the Philippines, and was seized just as the USNS Bowditch was about to retrieve it, U.S. officials said.

The Defense Ministry said a Chinese naval vessel discovered a piece of “unidentified equipment,” and checked it to prevent any navigational safety issues before discovering it was a U.S. drone.

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“China decided to return it to the U.S. side in an appropriate manner, and China and the U.S. have all along been in communication about it,” the ministry said on its website.

“During this process, the U.S. side’s unilateral and open hyping up is inappropriate, and is not beneficial to the smooth resolution of this issue. We express regret at this,” it added.

Trump, a Republican who takes office on Jan. 20, weighed into the dispute on Twitter on Saturday.

“China steals United States Navy research drone in international waters – rips it out of water and takes it to China in unprecedented act,” Trump said in a tweet early on Saturday from his seaside resort club Mar-a-Lago in Palm Beach, Florida, where he plans to spend the holidays.

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

For more on Trump, watch:

The Pentagon has said the drone was operating lawfully and was clearly marked as U.S. property.

Without directly saying whether the drone was operating in waters China considers its own, China’s Defense Ministry said U.S. ships and aircraft have for a long period been carrying out surveillance and surveys in “the presence” of Chinese waters.

“China is resolutely opposed to this, and demands the U.S. stops this kind of activity,” it said.

China will remain on alert for these sorts of activities and take necessary steps to deal with them, the ministry said without elaborating.

The Global Times, published by the ruling Communist Party’s official People’s Daily, earlier cited an unidentified Chinese source as saying they believed the issue would be resolved smoothly.

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But the drone incident has raised fresh concerns about China’s increased military presence and aggressive posture in the disputed South China Sea, including its militarization of maritime outposts.

New satellite imagery shows China has installed weapons, including anti-aircraft and anti-missile systems, on all seven artificial islands it has built in the South China Sea, a U.S. research group said this week.

Trump has previously complained about the South China Sea build-up. He has threatened to declare China a currency manipulator and force changes in U.S.-Chinese trade policy, which he says has led to the greatest theft of American jobs in history.

Trump has also raised questions about whether Washington would stick to its nearly four-decades-old policy of recognizing that Taiwan is part of “one China.”

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He irked Beijing by taking a congratulatory phone call from President Tsai Ing-wen of Taiwan after his Nov. 8 election victory. China lodged a diplomatic protest.

The 10-minute telephone call was the first of its kind since U.S. President Jimmy Carter switched diplomatic recognition from Taiwan to China in 1979, acknowledging Taiwan as part of “one China.”

President Barack Obama said on Friday it was fine for Trump to review Washington’s policy toward Taiwan, but he cautioned that a shift could lead to significant consequences in the U.S. relationship with Beijing.