The Pentagon is demanding that China return an “unlawfully seized” U.S. Navy underwater drone captured in international waters in the South China Sea, a confrontation likely to exacerbate tensions in a region where China is asserting greater control and expanding military installations.

A Chinese naval ship unlawfully seized the small unmanned vehicle Thursday while the USNS Bowditch, a U.S. Navy survey ship, was collecting it in a routine operation 50 nautical miles northwest of Subic Bay in the Philippines, according to a Defense Department statement.

“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 unmanned underwater vehicle is an unclassified “ocean glider” system used around world to gather data on salinity, water temperature and sound speed, the Pentagon said.