WASHINGTON – A Chinese ship on Thursday seized a small U.S. Navy underwater drone that had been operating in the South China Sea, triggering a formal diplomatic complaint from the U.S. government.

Two drones were collecting oceanographic information more than 50 miles off the coast of U.S. Naval Base Subic Bay in the Philippines when the survey vessel Bowditch attempted to retrieve them, U.S. officials said.

The crew had retrieved one drone when a Chinese ship that was shadowing the Bowditch swept in and seized the second, which was about 500 yards away, according to the officials.

The U.S. State Department has issued a formal demarche with Beijing about the incident and has demanded the return of the drone, which is known as an autonomous underwater glider.

“The vehicle was unlawfully seized,” one official said.

The seizure comes at a tenuous time in U.S.-China relations. China issued a strong condemnation of President-elect Donald Trump this week after he hinted his administration might be open to reevaluating four decades of U.S. policy toward China and Taiwan, which China regards as a rogue province.

The U.S. does not recognize Taiwan, but Trump earlier this month accepted a telephone call from Taiwan leader Tsai Ing-wen, setting off alarm bells in Beijing.

China’s Foreign Ministry warned that any changes could rupture diplomatic ties. Chinese state media called Trump “as ignorant as a child.”

Also escalating tensions is recent commercial satellite imagery showing that anti-aircraft guns and other weaponry has been installed on several of the artificial islands China is building off the mainland’s southern coast that sweep east of Vietnam, down near Malaysia and Brunei.

U.S. officials say the seized drone was collecting data on water salinity, surveying the ocean bottom and gathering other scientific information.

Submarine drones, often unarmed torpedo-like vehicles outfitted with radar and sensors, are useful in a variety of roles in science and national security.

Unlike aerial drones, which are remotely controlled using GPS signals and data links, robotic submarines are not able to receive satellite commands as they scour the ocean floor, sometimes for up to several months at a time.

So the machines need to be able to navigate on their own to carry out missions.

The unmanned subs can withstand the crushing pressures of the deep ocean, and have been used in ocean exploration and to locate airplane and shipwrecks