"If it is an HF radar, then it would enormously boost China’s capacity to monitor ships and aircraft in the South China Sea," Poling wrote by email. "Cuarteron is the logical place for such an installation because it is the southernmost of China’s features in the Spratlys, meaning that it would be the best place if you wanted early warning radar to give notice of ships or planes coming up from the Strait of Malacca and other areas to the south such as Singapore.

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"This would be very important in a Chinese anti-access area denial strategy that sought to reduce the ability of the U.S. to operate freely in the South China Sea, including bringing forces up through the South China Sea in case of any future crisis in Northeast Asia," Poling wrote.

The Strait of Malacca passes between Malaysia and Indonesia and is one of the most important shipping lanes in the world, while a third of the world's shipping, and much of Asia's oil, passes through the South China Sea.

China has built up seven islands in the South China Sea, and is in the process of constructing three runways on those islands. The United States says it is concerned about the growing militarization of the South China Sea, Secretary of State John F. Kerry expressed "serious concern" last week when other satellite images showed what appeared to be surface-to-air missile batteries deployed by China on Woody Island, part of the Paracel chain, also in the South China Sea.

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China says its construction program in the South China Sea is mainly for civilian use, adding that it is only building limited and necessary defensive facilities on what it considers to be its sovereign territory. It points out that other nations have also reclaimed land and built runways in the past, although not on anything like this scale.

"It is certainly possible to claim a civilian purpose, and China will," Poling wrote. "But just like you don’t need a 3,000-meter runway to land civilian planes, you don’t need a high-frequency radar (assuming that is what this is) to give early warning of commercial traffic. Radar is inherently dual-use, but just like its other “dual-use” infrastructure in the Spratlys, the real value is military. More limited radar, like China has at every other feature in the Spratlys, is more than sufficient to monitor and ensure the safety of civilian traffic near the features."

China points to lighthouses it has constructed on two islands, as well as meteorological stations and shelter and rescue facilities, to highlight the civilian nature of its construction program. One of the new lighthouses sits on Cuarteron Reef.

On Monday, Chinese Foreign Ministry spokeswoman Hua Chunying accused the United States of "sensationalizing the South China Sea issue" and "hyping up tensions."

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"Islands in the South China Sea have been part of China since ancient times," she said at a daily news conference. "The Chinese side is entitled to safeguard its territorial sovereignty and maritime rights and interests. China conducts construction on relevant islands and reefs mainly for civilian purposes of providing better public services and goods for the international community. China's deployment of limited defense facilities on its own territory is its exercise of self-defense right to which a sovereign state is entitled under international law. It has nothing to do with militarization. It is something that comes naturally, and is completely justified and lawful. The U.S. should view that correctly instead of making an issue of that with deliberate sensationalization."

Other photographs supplied to The Washington Post by CSIS also show radar facilities being built on other islands in the Spratlys, which are also claimed in full or in part by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.