By David Brunnstrom

WASHINGTON (Reuters) - Satellite images show China may be installing a high-frequency radar system in the Spratly Islands that could significantly boost its ability control the disputed South China Sea, a U.S. think tank reported on Monday.

The Asian Maritime Transparency Initiative at Washington's Center for Strategic and International Studies said the images showed that construction of facilities at Cuarteron Reef appeared nearly complete and that the artificial island now covered an area of about 52 acres (21 hectares).

China claims most of the South China Sea, through which more than $5 trillion in global trade passes every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.

"Two probable radar towers have been built on the northern portion of the feature, and a number of 65-foot (20-meter) poles have been erected across a large section of the southern portion," the report said.

"These poles could be a high-frequency radar installation, which would significantly bolster China's ability to monitor surface and air traffic across the southern portion of the South China Sea."

China's Ministry of Defense said in a statement on its microblog that the facilities China had established on the "relevant islands and reefs" included navigation and meteorological equipment.

"At the same time, we have deployed necessary defensive facilities on the islands," the statement added, adding the facilities were "legal and appropriate".

Chinese Foreign Ministry spokeswoman Hua Chunying, asked about the report, said she didn't know any details, but that China had every right to build on its own territory and deploy "limited" defensive means there.

The world should pay more attention to the civilian facilities China has built like lighthouses, Hua told a daily news briefing.

The report said the images showed that China appeared to have constructed a buried bunker and lighthouse on the northern portion of the feature, a helipad, communications equipment and a quay with a loading crane.

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The report, which based its analysis on satellite images from January and February, said China already had significant radar coverage of the northern part of the South China Sea given its mainland installations and in the Paracel Islands to the northwest of the Spratlys.

Both the U.S. and Vietnam last week expressed concern at China's deployment of advanced surface-to-air missiles on the Paracels.

A high-frequency radar on Cuarteron Reef would increase China's ability to monitor sea and air traffic coming north from the Malacca Straits and other important channels, the report said.

It also pointed to "probable" radars at Gaven, Hughes and Johnson South Reefs in the Spratlys as well as helipads, and to possible gun emplacements at the former two features.

China previously operated radars on Fiery Cross but the latest installations would give it far more extensive coverage, regional military analysts said.

The report comes a day before a visit to the United States by Chinese Foreign Minister Wang Yi, at which U.S. concerns about China's assertive territorial claims will be high on the agenda.

Beijing has been angered by air and sea patrols the United States has conducted near islands China claims in the South China Sea.

Hua said the United States was pushing "groundless worries" about freedom of navigation to carry out such patrols.

"The United States talks about freedom of navigation, but I fear in its heart what it's thinking about is absolute maritime hegemony," she added.

(Additional reporting by Greg Torode in HONG KONG and Ben Blanchard and Megha Rajagopalan in BEIJING; Editing by Lincoln Feast and Nick Macfie)