A US "naval glider" was reportedly being used to collect oceanographic data including salinity, clarity and water temperature, and was designed to assist in the mapping of underwater channels, a US defense official told the AFP news agency.

The incident took place just as the USNS Bowditch (pictured above), an oceanographic survey ship, was getting ready to retrieve the unmanned underwater drone, an official said speaking on condition of anonymity.

"It was taken" by China in international waters about 50 miles off Subic Bay west of the Philippines, the official added.

"It's a sovereign immune vessel, clearly marked in English not to be removed from the water - that it was US property," the official said.

The United States issued a formal demarche through diplomatic channels, demanding that China immediately return the underwater drone. Chinese officials have reportedly acknowledged the receipt of the demarche but have not responded to it.

Increasing militarization of South China Sea

The vessel's seizure came amid continued tensions in the South China Sea over Chinese military presence in the region. Earlier in the week, satellite images indicated that China had anti-aircraft and anti-missile systems installed on all seven artificial islands built in the South China Sea, as its stance on the disputed waters has grown increasingly aggressive.

The territories are under dispute by several other governments as well including Japan, Russia and the Philippines. China has steadily increased its militarization of the South China Sea and its island in recent years in an apparent bid to gain the upper hand in any territorial claims.

ss/sms (Reuters, AFP)