China "unlawfully" seized an unmanned US naval probe in international waters in the South China Sea, the Pentagon said on Friday (Dec 16), a move sure to heighten tensions around Beijing's military presence in the disputed area.

The underwater vehicle was taken around 90 kilometres northwest off Subic Bay in the Philippines late Thursday in a non-violent incident, said Captain Jeff Davis, a Pentagon spokesman.

The event unfolded as the civilian-crewed USNS Bowditch was retrieving a pair of "naval gliders" that routinely collect information on water temperatures, salinity and sea clarity.

A Chinese Dalang-III class submarine rescue ship then stopped within 500 metres of the Bowditch and snatched one of the probes. The Americans safely hoisted the other one back onto their ship.

Davis said he could not recall another time when something like this has happened, and the Pentagon issued a statement calling on Beijing to "immediately" return the probe that it had "unlawfully seized."

The US personnel "were asking over bridge-to-bridge radio to please leave it there," Davis said. Other than a brief acknowledgment that it had received the message, the Chinese ship did not respond.

"The only thing they said after they were sailing off into the distance was: 'We are returning to normal operations,'" Davis said.

Washington has issued a formal request through diplomatic channels to ask for the probe back. "It is ours. It is clearly marked as ours. We would like it back, and we would like this not to happen again," Davis said.

Pentagon press secretary Peter Cook said China had acted unlawfully.

"The UUV (unmanned underwater vehicle) is a sovereign immune vessel of the United States. We call upon China to return our UUV immediately, and to comply with all of its obligations under international law," Cook said in a statement.

HEIGHTENED TENSIONS

Davis said the seized vessel is off-the-shelf technology that is commercially available for about US$150,000. Data it gathers are unclassified and can be used to help submarines navigate and determine sonar ranges in murky waters.

The incident comes as President-elect Donald Trump has repeatedly infuriated Beijing, by questioning longstanding US policy on Taiwan, calling Beijing a currency manipulator and threatening Chinese imports with punitive tariffs.

"This was very likely a highly planned and escalatory move to show China will not take matters lightly when it comes to" Trump, said Harry Kazianis, the director of defence studies at the conservative Center for the National Interest.

"Beijing is showing it has the capability to respond in a time and place of its choosing."

Unless it is prepared to ramp up regional tensions, Washington has few options except to ask for the underwater vehicle back.

Senior Republican Senator John McCain said the United States should not tolerate "such outrageous conduct."

"This brazen provocation fits a pattern of increasingly destabilising Chinese behaviour, including bullying its neighbours and militarising the South China Sea," McCain said.

"This behaviour will continue until it is met with a strong and determined US response, which until now the Obama administration has failed to provide."

The incident comes amid broader tensions in the South China Sea, where China has moved to fortify its claims to the region by building out tiny reefs and islets into much larger artificial islands.

Brunei, Malaysia, the Philippines and Vietnam have competing claims in the South China Sea, which is laced with the world's most heavily travelled international trade routes.

New satellite imagery made public on Wednesday by a US-based think tank showed China apparently has installed what appeared to be large anti-aircraft guns and close-in weapons systems on seven islets in the Spratly chain.

While the United States takes no position on sovereignty claims in the South China Sea, it has repeatedly stressed all maritime claims must comply with international law.

The US military has conducted several "freedom of navigation" operations in which ships and planes have passed close to the sites Beijing claims.

Such missions have drawn howls of fury from China, which accuses Washington of provocation and increasing the risk of a military mishap.

The USNS Bowditch is a research and survey vessel, and does not look like a warship.

The Chinese ship's hull number was ASR-510, the Pentagon said.

(Channel NewsAsia)