'Let them keep it': Trump says U.S. doesn't want China to return drone 'they stole'

President-elect Donald Trump said Saturday the U.S. should let China keep the U.S. Navy's unmanned underwater glider that it seized in the South China Sea.

Trump tweeted: "We should tell China that we don't want the drone they stole back.- let them keep it!"

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The drone was seized on Thursday while collecting unclassified scientific data about 92 kilometres northwest of Subic Bay near the Philippines in the South China Sea, which China claims virtually in its entirety, Navy Capt. Jeff Davis, a Pentagon spokesman, said Friday.

"It is ours. It's clearly marked as ours. We would like it back, and we would like this not to happen again," Davis told reporters. He said the drone costs about $150,000 US and is largely commercial, off-the-shelf technology.

Trump blasted China for, as he described it, ripping the research drone out of the water.

The USNS Bowditch, which is not a combat ship, was stopped in international waters Thursday afternoon and recovering two of the gliders when a Chinese ship approached, Davis said. The two vessels were within about 450 metres of each other. He said the USNS Bowditch carries some small arms, but that no shots were fired.

According to the Pentagon, as the Chinese ship left with the drone, which is about 3 metres long, its only radio response to the U.S. vessel was, "We are returning to normal operations."

Bonnie Glaser, senior adviser for Asia at the Centre for Strategic and International Studies, said the seizure of the glider occurred inside the exclusive economic zone of the Philippines, not China, and appeared to be a violation of international law.

China delineates its South China Sea claims with a roughly drawn sea border known as the "nine-dash line" that runs along the west coast of the Philippines. However, it hasn't explicitly said whether it considers those waters as sovereign territory, and says it doesn't disrupt the passage of other nations' shipping through the area.

The U.S. doesn't take a position on sovereignty claims, but insists on freedom of navigation, including the right of its naval vessels to conduct training and other operations in the sea.

China to return glider after 'unlawful seizure'

Trump's tweet came as the U.S. military said that, through "direct engagement" with the Chinese, it's "secured an understanding" that China's navy will return the underwater glider.

Peter Cook, a spokesman for U.S. Defense Secretary Ash Carter, said in a statement that the U.S. had registered its objection to what it's calling an "unlawful seizure" of the unmanned submerged device in international waters.

China says its navy seized the glider to ensure the "safe navigation of passing ships."

"In order to prevent this device from posing a danger to the safe navigation of passing ships and personnel, the Chinese lifeboat adopted a professional and responsible attitude in investigating and verifying the device," China's Defense Ministry spokesman Yang Yujun said in a statement on Saturday.

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It also accused the U.S. of deploying ships in China's presence to conduct military surveying. "China is resolutely opposed to this and requests the U.S. stop such activities," the statement said.

Relations were already tense between the U.S. and China following Trump's decision to talk by phone with Taiwanese President Tsai Ing-wen on Dec. 2. He later said he did not feel "bound by a one-China policy" regarding the status of Taiwan, 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.