Satellite images showed two batteries of eight surface- to-air HQ-9 missile launchers as well as a radar system on Woody Island, part of the Paracel Island chain in the South China Sea, Fox News reported.
The report comes even as US President Barack Obama called for "tangible steps" to settle territorial disputes in the resource-rich region.
According to the images, a beach on the island was empty on February 3, but the missiles were visible by February 14.
It is the same island where a US Navy destroyer sailed close to another contested island a few weeks ago. Woody Island is part of the Paracels chain, under Chinese control for more than 40 years also claimed by Taiwan and Vietnam. The missiles arrived on the island over the past week.
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China decribed the report as media hype.
"We believe this is an attempt by certain Western media to create news stories," Chinese Foreign Minister Wang Yi said.
"All of those are actions that China, as the biggest littoral state in the South China Sea, has undertaken to provide more public goods and services to the international community and play its positive role there," he said.
Wang said China's construction of military infrastructure was "consistent with the right to self-preservation and self- protection that China is entitled to under international law, so there should be no question about that."
The US will continue to fly, sail and operate wherever international law allows, and will support the right of all countries to do the same, Obama said yesterday, as he called for "tangible steps" to reduce tensions in the disputed and natural resource-rich South China Sea.