WASHINGTON (Reuters) – A Chinese Navy warship has seized an underwater drone deployed by an American oceanographic vessel in international waters in the South China Sea, triggering a formal diplomatic protest from the United States and a demand for its return, a U.S. defense official told Reuters on Friday.

The incident, the first of its kind in recent memory, took place on Dec. 15 northwest of Subic Bay off the Philippines just as the USNS Bowditch, an oceanographic survey ship, was about to retrieve the unmanned, underwater vehicle (UUV), the official said, speaking on condition of anonymity.

“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 U.S. property.”

The Chinese seizure will add to concerns about China’s growing military presence and aggressive posture in the disputed South China Sea, including its militarization of maritime outposts.

A U.S. think tank reported this week that new satellite imagery indicated that China has installed weapons, including anti-aircraft and anti-missile systems, on all seven artificial islands it has built in the South China Sea.

The seized underwater drone was part of an unclassified program to collect oceanographic data, including salinity, temperature and clarity of the water, the official added.

Such data can help inform U.S. military sonar data, since sound is affected by such factors.

The United States issued the formal demarche, as such protests are known, through diplomatic channels and included a demand that China immediately return the underwater drone.

The Chinese have acknowledged the demarche but not responded to it, the official added.

(Reporting by Phil Stewart; Editing by Chizu Nomiyama and James Dalgleish)

IMAGE: The USNS Bowditch, an oceanographic survey ship, is seen in this undated U.S. Navy handout photo. U.S. Navy via REUTERS