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[Nagasdog]

China claims almost all the waters, which carry a third of the world's maritime traffic. Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims. Trump's administration has called China's island building in the South China Sea illegal.

Building the concrete structures with retractable roofs on Subi, Mischief and Fiery Cross reefs, part of the Spratly Islands chain where China already has built military-length airstrips, could be considered a military escalation, the U.S. officials said in recent days, speaking on condition of anonymity.

"It is not like the Chinese to build anything in the South China Sea just to build it, and these structures resemble others that house SAM batteries, so the logical conclusion is that's what they are for," said a U.S. intelligence official, referring to surface-to-air missiles.

Another official said the structures appeared to be 20 meters (66 feet) long and 10 meters (33 feet) high.