We call upon China to return our UUV immediately," Pentagon Press Secretary Peter Cook demanded.
The US on Friday asked China to "immediately" return its unmanned naval probe in international waters in the South China Sea which it alleged has been "unlawfully" seized by them.
"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 December 15 in the South China Sea while it was being recovered by a US Navy oceanographic survey ship," Pentagon Press Secretary Peter Cook said.
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 PRC DALANG III-Class ship (ASR-510) launched a small boat and retrieved the UUV, he said.
"Bowditch made contact with the PRC Navy ship via bridge-to-bridge radio to request the return of the UUV. The radio contact was acknowledged by the PRC Navy ship, but the request was ignored," he said.
"The UUV is a sovereign immune vessel of the US. We call upon China to return our UUV immediately, and to comply with all of its obligations under international law," Mr Cook demanded.
Senator Ben Cardin, Ranking Member of the Senate Foreign Relations Committee said that China's seizure of an US Navy unmanned scientific research submersible in international waters off the South China Sea is a remarkably brazen violation of international law.
"No matter its motivation, China needs to explain its actions and immediately return the undersea drone, untampered with, to the United States," he said.
"China's increasingly aggressive behaviour in the South China Sea - including the militarisation of the artificial features that it has built in the region, in direct contradiction of a pledge made by President Xi Jinping not to do so - is deeply disturbing, and not consistent with the actions of a nation seeking to build a constructive and cooperative relationship with the United States," Mr Cardin said.
"The United States has a deep and abiding interest in freedom of navigation and the free-flow of commerce in the Asia-Pacific region, and a stead-fast commitment to our regional allies and partners in maintaining stability, security and the rule of law, including safe passage in international waters," Mr Cardin said.
The incident comes amid rising tensions in the South China Sea due to Beijing's military presence in the disputed area.