Donald Trump has accused China of "stealing" a US Navy drone, after it was seized in international waters of the South China Sea.
The President-elect tweeted: "China steals United States Navy research drone in international waters – rips it out of water and takes it to China in unpresidented [sic] act."
The drone's seizure, which followed Mr Trump's call with the president of Taiwan, was described as one of the most serious diplomatic incidents between the two nations in recent memory.
The message risked a re-escalation in tensions over a number of issues between the two countries, which officials had been working to repair since the incident.
China's Defence Ministry said after Mr Trump's tweet that it had been in touch with the Washington about the drone, but the US's "hyping up" of the issue was not conducive to resolving the issue.
Some interpreted China's decision to take the drone as a warning signal to Mr Trump because of his warmth towards Taiwan.
In taking a congratulatory call from Taiwanese President Tsai Ing-wen, Mr Trump broke with a long-standing diplomatic practice adopted by the US.
As part of its foundational 'One China' policy, Beijing regards the island as part of its sovereign territory.
Only on Friday, Barack Obama said: “If you are going to up-end this understanding, you have to have thought through what are the consequences."
The President warned Mr Trump against slipping into "full conflict mode" with the country.
“Because the Chinese will not treat that [Taiwan] the way they will treat some other issues. They won’t even treat it the way they treat issues around the South China Sea, where we have had a lot of tensions. This goes to the core of how they see themselves and their reaction on this issue could end up being very significant," Mr Obama added.
Some observers on social media also mocked the President-elect's misspelling of 'unprecedented'.
The tweet was later resent, apparently by Mr Trump's staff, with the correct spelling.
In a statement issued before Mr Trump's tweet, China's foreign ministry had said: "According to [our] understanding, the US and Chinese sides are working on appropriately handling this matter through channels between the two militaries."
And a newspaper published by China's ruling Communist Party also cited a military officer as saying that a "smooth resolution" of the matter was expected.
According to the US, the drone was carrying out scientific research underwater and was controlled by civilian contractors.
US officials had issued a formal diplomatic complaint to China demanding the drone's return.
"It is ours. It's clearly marked as ours. We would like it back, and we would like this not to happen again," Pentagon spokesman Captain Jeff Davis told reporters.
The drone costs about $150,000 (£120,000) and is largely commercial, off-the-shelf technology, Capt Davis added.
The USNS Bowditch, which is not a combat ship, was stopped in international waters on Thursday afternoon while attempting to recover two of its gliders when the Chinese ship approached, Capt Davis said. The Chinese ship then took one drone away.
According to the Pentagon, as the Chinese ship left with the drone, which is about three meters long, its only radio response to the US vessel was, "we are returning to normal operations".
Bonnie Glaser, senior adviser for Asia at the Center 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 territorial 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 US 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.
Additional reporting by AP and Reuters
