WASHINGTON — A Chinese Navy ship intercepted and grabbed a small, unmanned underwater vehicle (UUV) being operated by a US Navy survey ship on Thursday in waters west of the Philippines, US defense officials confirmed Friday.

It is not clear what — if anything — prompted the interception of an ocean glider, described by the National Oceanic and Atmospheric Administration (NOAA) as "an autonomous, unmanned underwater vehicle used for ocean science."

Ocean gliders, according to the NOAA website, "may be equipped with a wide variety of sensors to monitor temperature, salinity, currents, and other ocean conditions." The gliders are not driven by propellers, but use "large moveable fins and a large on-board battery pack to gently change its buoyancy over time." The system is a commercially-based system. 

The Bowditch is a US Navy survey ship operated by the Military Sealift Command and crewed by civilian mariners. The ship carries out military and civilian missions, and in September it took part in amphibious exercises with Philippine forces.

A US defense official said the Bowditch was "conducting a routine military survey in international waters." He noted the glider "is an unclassified system used to collect oceanographic data such as temperature, salinity, sound speed and water clarity."

In the incident — reported first by CNN — the Bowditch was in the process of recovering two ocean gliders Dec. 15 around noon local time about 50 nautical miles northwest of Subic Bay on the Philippine island of Luzon. The first glider had been brought aboard when, according to the US defense official, the Chinese ship slipped in and intercepted the second glider.

Pentagon press secretary Peter Cook issued a statement Friday afternoon calling upon the Chinese government to immediately return the drone.

"Using appropriate government-to-government channels, the Department of Defense has called upon China to immediately return an unmanned underwater vehicle (UUV) that China unlawfully seized on Dec. 15 in the South China Sea while it was being recovered by a U.S. Navy oceanographic survey ship," Cook said in the statement.

"The USNS Bowditch (T-AGS 62) and the UUV -- an unclassified "ocean glider" system used around the world to gather military oceanographic data such as salinity, water temperature, and sound speed - were conducting routine operations in accordance with international law about 50 nautical miles northwest of Subic Bay, Philippines, when a Chinese Navy [People's Republic of China] DALANG III-Class ship (ASR-510) launched a small boat and retrieved the UUV.

"Bowditch made contact with the PRC Navy ship via bridge-to-bridge radio to request the return of the UUV," Cook continued. "The radio contact was acknowledged by the PRC Navy ship, but the request was ignored. The UUV 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."

The Chinese ship was roughly 500 yards away from the Bowditch when the incident occurred, said Capt. Jeff Davis, Pentagon spokesman. Davis added that a crane was used to lift the unmanned system aboard the Chinese vessel.

The defense official noted the location in the South China Sea was not in the proximity of Scarborough Shoal, the site of a disputed Chinese island-building operation. "It’s not even close to Scarborough. It’s about 150 miles away," the defense official said.

It's unclear what the Dalang 510 did after seizing the ocean glider. The Bowditch, the defense official said Friday, "remains in the area conducting normal operations."

Davis said there were no shots fired by the US or Chinese ships, but that the US ship attempted to demand the Chinese ship release the system.

"There were multiple attempts made over bridge-to-bridge radio to demand it back. They acknowledged the communications on bridge-to-bridge radio. It wasn’t a radio problem. But [they] ignored the request for it to be returned," Davis said of the incident. "It's certainly not something that we consider to be commensurate with their level of professionalism."

The US has filed a démarche with the Chinese government, protesting the incident.

US Sen. Ben Cardin of Maryland, the top Democrat on the Senate Foreign Relations Committee and its Subcommittee on East Asia and the Pacific, called the seizure, "a remarkably brazen violation of international law," and demanded China immediately explain its actions and return the drone untampered with to the US. In a statement, Cardin, D-Md., called out China's "increasingly aggressive behavior in the South China Sea" and "militarization of the artificial features that it has built in the region."

This is not the first time Chinese vessels have tried to grab operating equipment from a US Navy ship. On March 8, 2009, several Chinese trawlers operated by naval militia harassed the US Navy’s intelligence ship Impeccable about 70 miles south of Hainan Island in the South China Sea, attempting and failing to cut off and steal its towed sonar array.

Earlier this year, the US defense official said, an "unknown vessel" grabbed another underwater vehicle operated by a US ship near Vietnamese waters, but the vehicle was recovered. No further details were provided.

While harassment of operations by Chinese naval militia are common in the region, it is unusual for a Chinese Navy ship to make such a move.

Joe Gould and Aaron Mehta in Washington contributed to this report

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