According to CNBC:

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.