By J.R. Wu

TAIPEI (Reuters) - Taiwan's Defence Ministry said it was asking Google to blur satellite images showing what experts say appear to be new military installations on Itu Aba, Taipei's sole holding in the hotly disputed South China Sea.

The disclosure of new military-related construction could raise tensions in the contested waterway, where China's building of airstrips and other facilities has worried other claimants and the United States.

The images seen on Google Earth show four three-pronged structures sitting in a semi-circle just off the northwestern shoreline of Itu Aba, across from an upgraded airstrip and recently constructed port that can dock 3,000-ton frigates.

"Under the pre-condition of protecting military secrets and security, we have requested Google blur images of important military facilities," Taiwan Defence Ministry spokesman Chen Chung-chi said on Wednesday, after local media published the images on Itu Aba.

The United States has urged against the militarization of the South China Sea, following the rapid land reclamation by China on several disputed reefs through dredging and building airfields and port facilities.

A spokesman for the U.S. Defense Department, Commander Gary Ross, said the Pentagon was aware of the reports of the military-related construction and encouraged all claimants to take steps to lower tensions.

"We believe a reciprocal halt among all claimants, including Taiwan, on any further land reclamation, construction, and militarization of land features would lower tensions," he said.

Taiwan's Defence Ministry and coast guard, which directly oversees Itu Aba, said details about the structures were confidential and have not commented on their nature.

Google, a unit of Alphabet Inc, said it was reviewing the request from Taiwan, but that such requests to date had not resulted in Google blurring imagery.

Satellite images used on Google Earth and Google Maps come from third-party providers, which Google purchases for use in its mapping system, according to the search giant.

"We take security concerns very seriously, and are always willing to discuss them with public agencies and officials," Google spokesman Taj Meadows said in an emailed response.

Defence experts in Taiwan said that based on the imagery of the structures and their semi-circular layout, the structures were likely related to defense and could be part of an artillery foundation.

"I think definitely it will be for military purposes, but I cannot tell if it is for defending, attacking or monitoring," said Dustin Wang, a scholar and a former government adviser who has regularly visited Itu Aba.

Wang said given the structures' location, which faces the main seaborne traffic, they may relate to surveillance.

China, Taiwan, the Philippines, Vietnam, Malaysia and Brunei claim parts or all of the South China Sea, through which trillions of dollars in trade passes.

In July, an international court ruled against China in a case brought by the Philippines that rejected China's claim to a vast swathe of the disputed maritime area. Both China and Taiwan, which China views as a renegade province, vehemently rejected the court ruling.

(Additional reporting by David Brunnstrom in New York; Editing by Lincoln Feast and Peter Cooney)