China to hold military drills amid South China Sea tensions
Chinese dredging vessels are purportedly seen in the waters around Mischief Reef in the disputed Spratly Islands in the South China Sea Reuters

South China Sea territory tussle took another nasty turn on Friday after a Chinese warship seized a US Navy underwater drone. Washington asked China to return the drone immediately and US officials said they could not recall a similar event in the past.

"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.

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The drone was seized about 50 nautical miles northwest of Subic Bay off the Philippines. The US said China acted unlawfully. "The UUV was lawfully conducting a military survey in the waters of the South China Sea," an unnamed US official said, Reuters reported.

Even as U.S. Navy's oceanographic ship the USS Bowditch was collecting information on water temperature from two gliders one of these craft was seized by China, Pentagon spokesman Jeff Davis said.

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"The UUV [unmanned underwater vehicle] was lawfully conducting a military survey in the waters of the South China Sea," he said at a press briefing.

The underwater drone was seized by a Dalang-III class submarine rescue ship which came within 500 metres of the US vessel when it took away the probe.

Even as the US personnel asked the Chinese to leave the drone, they 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.

"The UUV (unmanned underwater vehicle) is a sovereign immune vessel of the United States. We call upon China to return our UUV immediately, and to comply with all of its obligations under international law," Cook said, according to the CNN.

The latest confrontation in the disputed waters took place after media reports said China built advanced weapons systems in all seven artificial islands it developed around the disputed Spratly island in the South China Sea.

The latest defence facilities erected in the artificial islands are most likely anti-aircraft guns or weapon systems to thwart cruise missile strikes, reports said. China had already built airstrips on the artificial islands, much to the consternation of countries in the region. The installation of advanced weaponry is aimed at defending the islands in case of a missile attack, think tank Asia Maritime Transparency Initiative said.