US navy patrol boat (file)
US navy patrol boat (file)Reuters

A US drone deployed by an oceanographic vessel in the South China Sea was captured by China northwest of the Subic Bay on Thursday just before its retrieval by a US ship.

The drone, which was part of an unclassified program to collect oceanographic data in the South China Sea's international waters, was defended by US officials who said, "The UUV was lawfully conducting a military survey in the waters of the South China Sea. It’s a sovereign immune vessel, clearly marked in English not to be removed from the water — that it was US property."

Pentagon spokesman and Navy Captain Jeff Davis said this may be the first time in recent history China took a US naval vessel. Though the Chinese ship did acknowledge a radio call from the US ship, it did not respond.

"The only thing they said after they were sailing off into the distance was, ‘We are returning to normal operations,'" Davis said.

Meanwhile, the Pentagon has demanded China return the "unlawfully seized" drone.

"We call upon China to return our UUV immediately, and to comply with all of its obligations under international law," Pentagon press secretary Peter Cook said in a statement.

A UUV is an unmanned underwater vehicle.

China's Defense Ministry and the Pentagon have since said China will return the drone to the US navy.

"Through direct engagement with Chinese authorities, we have secured an understanding that the Chinese will return the UUV to the United States," Cook said in a statement.

However, China expressed clear disapproval of the fact the US publicized the drone's capture, explaining the drone was seized after the Chinese navy noticed an "unidentified" piece of equipment. They also said China had checked the drone for navigational safety issues before realizing it belonged to the US.

"China decided to return it to the U.S. side in an appropriate manner, and China and the U.S. have all along been in communication about it," wrote China's Defense Ministry on its website. "During this process, the U.S. side's unilateral and open hyping up is inappropriate, and is not beneficial to the smooth resolution of this issue. We express regret at this," it added.

China also said they opposed the presence of US ships and aircraft in Chinese waters.

"China is resolutely opposed to this, and demands the U.S. stops this kind of activity," it said, noting the ministry would take necessary steps to deal with the intrusions.

Last July, China rejected a Hague Court ruling which said China has no historic claim over the South China Sea, and in 2015 criticized the US for "meddling" in affairs in the area.