China has now agreed to return a US submarine drone taken on Thursday, but the incident could be a harbinger of tense relations between the US and China under the new administration.

Moments after the glider was seized by a Chinese ship, the US began requesting its return. On Friday, the Pentagon issued a diplomatic complaint and demanded the return of the drone, saying China’s actions violated international law. President-elect Donald Trump weighed in Saturday morning, echoing concerns about the seizure.

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

China agreed on Saturday to return the drone, the Defense Ministry said on its website.

“China decided to return it to the US side in an appropriate manner, and China and the US have all along been in communication about it,” said a statement by China's defense ministry, according to Reuters and the Associated Press.

The statement also accused the US of long deploying ships "in China's presence" to conduct "military surveying."

"China is resolutely opposed to this and requests the US stop such activities," it said. "China will continue to maintain vigilance against the relevant US activities and will take necessary measures to deal with them."

Some have suggested that China's seizure of the drone is Beijing sending a message to Trump about the future of the US role in Asia.

“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 US-China relations,” said Ashley Townshend, research fellow at the US studies center at the University of Sydney, Bloomberg reported.

A Chinese ship seized the unmanned Navy glider on Thursday about 50 nautical miles northwest of the Philippines, said Navy captain and Pentagon spokesman Jeff Davis on Friday, according to the Associated Press. The glider, which collects unclassified data about the temperature, clarity, and salinity of the water for use in sonar operations, was about to be retrieved by the civilian-operated USNS Bowditch when it was taken by the Chinese.

The Chinese ship reportedly discovered that the piece of “unidentified equipment” it had picked up was a US drone during safety checks. But the Pentagon said Friday that the USNS Bowditch had repeatedly contacted the Chinese ship to request the glider’s return – requests that went unanswered.

The glider was seized inside the Philippines’ exclusive economic zone, said Bonnie Glaser, senior adviser for Asia at the Center for Strategic and International Studies, Al Jazeera reported. As a result, the seizure appeared to be a violation of international law.

Official Chinese discussion of the incident has been brief. In a statement, China’s foreign ministry said, “According to [our] understanding, the US and Chinese sides are working on appropriately handling this matter through channels between the two militaries,” according to Al Jazeera. A statement sent to Bloomberg on Saturday echoed that remark.

Some suggested that the drone’s activity was perceived as a threat.

Unmanned underwater vehicles “can track our nuclear ballistic missile submarines fleet,” said retired Major General Xu Guangyu, a senior researcher at the China Arms Control and Disarmament Association, a Beijing-based research group, according to Bloomberg. “If one from the Bowditch can be detected and even snatched by a Chinese naval ship, it shows it’s getting too close to the sensitive water areas.”

Others argued that China was entitled to seize and investigate the drone.

“If China needs to take it, we’ll take it,” said retired admiral Yang Yi, Al Jazeera reported. He called it “natural” for China to seize and research technologies that “America sends to our doorstep.”

The incident deserves a reaction, say some in the United States. Senator Cory Gardner (R) of Colorado, who chairs the Senate’s Foreign Relations subcommittee for East Asia, called it “outrageous” and demanded “a firm response,” reported The Hill. President-elect Trump called the act “unprecedented,” while Senator John McCain (R) of Arizona suggested that China was demonstrating a willingness to disrupt the stability of the region.

It’s unclear what the incident will mean going forward. But it appears to be “highly escalatory” move, said Mira Rapp-Hooper, a senior fellow in the Asia-Pacific Security Program at the Center for a New American Security, Reuters reported.

And it comes at a time when the relationship is already stretched. On the campaign trail, Trump blamed China for American job losses, and this month he became the first US leader to officially speak with Taiwan’s leader since 1979, when the US embarked on its “One China” policy. China reportedly considered the phone call an affront by the US.

China indicated on Thursday that it had militarized the islands it built in the South China Sea, the New York Times reported, reinforcing US concerns about heightened aggression in the region.

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With the Sino-US relationship apparently uncertain as President Obama leaves office, China may be looking to set the tone for the next four years, suggested Michael Fuchs, deputy assistant secretary of state for East Asian and Pacific Affairs from 2013 to 2016 and now a senior fellow at the Center for American Progress.

“It’s entirely possible that the Chinese are beginning to look for ways to send signals to the new administration that they are not going to be pushed around when it comes to the US role in Asia, whether it’s Taiwan or the South China Sea,” he told Bloomberg.