President-elect Donald Trump says the United States should let China keep the US Navy’s unmanned underwater glider that it seized in the South China Sea.

“We should tell China that we don’t want the drone they stole back _ let them keep it!” Mr Trump tweeted a few hours after the US military announced it had reached an understanding with China for the return of the underwater glider.

According to the Pentagon, the drone was seized on Thursday while collecting unclassified scientific data in the South China Sea, which China claims virtually in its entirety.

The US demanded the drone back, calling it an “unlawful seizure” in international waters.

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

— Donald J. Trump (@realDonaldTrump) December 18, 2016

China said its military seized the underwater glider to ensure the safe navigation of passing ships but would give it back.

It was not immediately known what effect, if any, the president-elect’s tweet would have on the agreement with the Chinese.

The evening tweet was the second time the president-elect injected himself into the controversy through Twitter on Saturday.

Misspelling “unprecedented,” he tweeted on Saturday morning: “China steals United States Navy research drone in international waters – rips it out of water and takes it to China in unpresidented act.”

He later reissued the tweet, correcting the spelling.

Relations already were tense between the US and China following Trump’s decision to talk by phone with Taiwanese President Tsai Ing-wen on December 2.

Mr Trump later said he did not feel “bound by a one-China policy” regarding the status of Taiwan, unless the US 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.

China says Pentagon hype has not helped resolve the issue. Photo: Getty

China’s Defence Ministry had said it was in talks with the US about returning the drone, but scoffed at US hype over the issue.

The drone was taken on Thursday — the first seizure of its kind in recent memory — about 50 nautical miles north-west of Subic Bay off the Philippines, just as the USNS Bowditch was about to retrieve the unmanned underwater vehicle (UUV), US officials said.

The Pentagon says the UUV was collecting oceanographic data.

“Through direct engagement with Chinese authorities, we have secured an understanding that the Chinese will return the UUV to the United States,” Pentagon spokesman Peter Cook said in a statement on Saturday.

Earlier in the day China’s Defence Ministry said it was in talks with the United States about returning the drone taken by a Chinese naval vessel, but the US was not helping by “hyping up” the issue.

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 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,” the Defence Ministry said on its website.

“During this process, the US 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.

The United States says the drone was operating lawfully.

The Pentagon confirmed the incident at a news briefing on Friday, and said the drone used commercially available technology and sold for about $US150,000 ($A205,080).

Still, the Pentagon viewed China’s seizure seriously since it had effectively taken US military property.

“It is ours, and it is clearly marked as ours and we would like it back. And we would like this not to happen again,” Pentagon spokesman Jeff Davis said.

With AAP