US President-elect Donald Trump speaks during a USA Thank You Tour event in Orlando, Florida, U.S., December 16, 2016. REUTERS/Lucas Jackson President-elect Donald Trump tweeted Saturday evening that China should be told to "keep" the US Navy underwater drone it recently seized from South China Sea waters and agreed to return.

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

Earlier on Saturday, Trump had called the capture — an incident that added to existing tensions between the US and China — an "unprecedented" act.

"China steals United States Navy research drone in international waters — rips it out of the water and takes it to China in an unprecedented act," he tweeted, after deleting a widely mocked version of the tweet that used the misspelling "unpresidented."

The drone, deployed by a US oceanographic vessel in international waters in the South China Sea, was conducting a military survey roughly 50 nautical miles northwest of the Philippines before it was seized by a Chinese warship and removed from the water on Thursday.

"It's a sovereign immune vessel, clearly marked in English not to be removed from the water — that it was US property," a US official told Reuters on Friday.

Chinese officials agreed to return the vessel after negotiating with the US, the Pentagon said Saturday.

"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.

However, China's Defense Ministry said it was "resolutely opposed" to this kind of activity, and accused the US of "hyping up" the incident.

Navy file photo of the T-AGS 60 Class Oceanographic Survey Ship, USNS Bowditch, the ship that launched the drone. US Navy

China's seizure of the vessel comes amid an increased military presence in the South China Sea.

A US think tank said Friday that China has installed antiaircraft and antimissile systems on man-made islands it has been constructing around the Spratly Islands, a strategically important chain that China occupies.

Trump, a president-elect with no formal diplomatic experience, has taken stances opposing China's interests since winning the election in November.

He held a congratulatory phone call with the Taiwanese prime minister, Tsai Ing-wen, earlier this month, flouting decades of established US policy.

Trump has also called China a "currency manipulator," and threatened to impose a 45% tariff on Chinese imports on the campaign trail.

The president-elect was set to continue his "thank you" tour of postelection rallies in Alabama on Saturday.