A Chinese warship seized an underwater drone deployed by an American oceanographic vessel in international waters in the South China Sea — triggering a US protest and demand for its return, officials said.

The incident took place Thursday northwest of Subic Bay off the Philippines as the US Navy ship USNS Bowditch — which has a civilian crew — was about to retrieve the ­unmanned, underwater vehicle, Reuters reported.

The drone was part of an unclassified program to collect oceanographic data, including salinity, temperature and other scientific facts used for sonar operations, a US official said.

“The UUV was lawfully conducting a military survey in the waters of the South China Sea,” the official said. “It’s a sovereign immune vessel, clearly marked in English not to be removed from the water — that it was US property.”

Navy Capt. Jeff Davis, a Pentagon spokesman, said the US ­issued a formal complaint but he was unaware of any ­response. He said this may be the first time in recent history that China has taken a US naval vessel.

The Bowditch had already retrieved another drone and was 500 yards from the seized device when a Chinese Dalang ­III-class vessel grabbed it.

The Chinese ship acknowledged a radio call from the Bowditch but did not respond to the US demands, Davis said.

“The only thing they said after they were sailing off into the distance was, ‘We are returning to normal operations,’ ” he said. No shots were fired, he added.

The Pentagon said the 10-foot-long drone used commercially available technology and sold for about $150,000, but it stressed that China had seized US military property.

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

The Chinese action heightened US concerns about the country’s tactics in the disputed sea — including its militarization of maritime outposts.

An American think-tank reported this week that new satellite images showed that China has installed weapons — including anti-aircraft and anti- missile systems — on all seven artificial islands it has built in the sea, Reuters reported.

The incident comes after President-elect Donald Trump has questioned the United States’ “one China” foreign policy, in which formal relations are kept with Beijing and not Taiwan.

With Post Wires