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The Pentagon said Saturday that China will return a U.S. Navy drone seized in the international waters of the South China Sea.

"We have registered our objection to China's unlawful seizure, of a U.S. unmanned-underwater vehicle,” Pentagon spokesman Peter Cook said. “Through direct engagement with Chinese authorities, we have secured an understanding that the Chinese will return the UUV to the United States."

President-elect Donald Trump tweeted the following Saturday night:

Earlier Saturday, Trump sent a tweet that accused the Chinese of stealing the drone and ripping “it out of water.”

He tweeted at about 7:30 a.m. Saturday.

Trump, a Republican, called the seizure Thursday “unpresidented,” though he needed to delete and replace the original tweet to correct the spelling of the word.

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China's seizure is being considered by some observers as one of the most significant disputes in years between the world super powers and follows Trump’s post-election victory call with the president of Taiwan.

The call alarmed old guard foreign policy experts who said it undermined the United States’ decades-long “One China” policy that recognizes mainland China over Taiwan.

Before the Pentagon announcement, China had indicated that efforts were underway to resolve the issue, saying it was in contact with American counterparts regarding the "appropriately handling" of the unmanned underwater glider.

However, the one-sentence comment by the country’s foreign ministry offered no details on what discussions were underway or why China on seized the drone, which the Pentagon says was being operated by civilian contractors for oceanic research.

The U.S. said it issued a formal diplomatic complaint over the seizure and demanded the drone's return.

"According to (our) understanding, the U.S. and Chinese sides are working on appropriately handling this matter through channels between the two militaries," the foreign ministry said in its statement, which was faxed to The Associated Press.

China's defense ministry did not immediately respond to questions on the issue.

The drone was seized while collecting unclassified scientific data about 57 miles northwest of Subic Bay, near the Philippines in the South China Sea, which China claims virtually in its entirety, the U.S. Navy said Friday.

"It is ours,” a Navy spokesman said. “It's clearly marked as ours. We would like it back, and we would like this not to happen again,"

The spokesman also said the drone costs about $150,000 and is largely commercial, off-the-shelf technology.

The most recent, significant dispute between the countries’ militaries was perhaps in April 2001 when a U.S. Navy surveillance aircraft and a Chinese fighter jet nearly collided about 70 miles from China's Hainan island, which led to the death of a Chinese pilot.

Whatever the outcome of the drone incident, the incident is likely to fray the already tense relations between U.S. and China.

Beijing was angered by Trump's decision to talk Dec. 2 by phone with Taiwanese President Tsai Ing-wen and by his later comments that he did not feel "bound” by the One China policy, unless the U.S. could gain trade or other benefits from China.

China considers the self-governing island its own territory to be recovered by force if it deems necessary.

There also have been increased tensions over Beijing's ongoing military buildup in the South China Sea, mainly the development and militarization of man-made shoals and islands aimed at extending China's reach in the strategically vital area through which about $5 trillion in global trade passes annually.

In one of the few reports in state media about the drone's seizure, a newspaper published by China's ruling Communist Party cited an unidentified military official as saying that a "smooth resolution" to the matter is expected.

A Chinese navy ship discovered an "unidentified device" Thursday and was checking on it for the sake of maritime safety, the Global Times quoted the official as saying.

"China has received the U.S. request to return the device, communication is open between the relevant departments of the two sides and I believe this matter will obtain a smooth resolution," the officer was quoted as saying.

In a separate report, the paper quoted retired Chinese admiral Yang Yi as saying China considered itself well within its rights to seize the drone.

"If China needs to take it, we'll take it. (America) can't block us," Yang was quoted as saying.

Yang said he was unsure of the purpose of seizing the drone, but didn't think the matter qualified as a "military conflict." However, he added that the chances of a confrontation had risen following Trump's recent comments, which were seen as testing China's bottom line on Taiwan and other sensitive issues.

"It's natural for us to take possession of and research for a bit these types of things that America sends to our doorstep," Yang said. "The louder they shout, the more their protests ring hollow."

The Associated Press contributed to this report.