A Chinese Navy warship has seized a vessel of the United States in the international waters of the South China Sea. The underwater drone was deployed by oceanographic vessel USNS Bowditch northwest of Subic Bay, off the coast of the Philippines. It was seized from China on Thursday minutes before the unmanned, underwater vehicle (UUV) was going to be retrieved, an official told Reuters on Friday.

“The UUV was lawfully conducting a military survey in the waters of the South China Sea,” the official said. “It is a sovereign immune vessel, clearly marked in English not to be removed from the water and that it was US property.” The drone was part of a program to collect oceanographic data, including temperature, clarity and salinity of the water, the official added.

Following the seizure, the US has demanded the immediate release and return of the vessel, in a formal demarche lodged through diplomatic channels. China has yet to respond to the communication, though Beijing has acknowledged it, the official said.

The news comes a day after China admitted that it had installed weapons systems on the artificial islands it had built on the disputed South China Sea. However, the country’s Ministry of Foreign Affairs said the deployment was part of normal defence process and should not be seen as “militarisation”.

In July, an international tribunal in The Hague had ruled in favour of the Philippines and said Beijing had no legal basis to claim “historic rights” over the South China Sea. The Permanent Court of Arbitration accused the country of breaching the sovereign rights of the Philippines by exploring resources near the Reed Bank. However, China dubbed the verdict “ill-founded”. Beijing has also said that its “territorial sovereignty and marine rights” in the seas will not be affected by the verdict.