Taiwan has asked Google to blur images of new military structures that have recently appeared on the hotly contested Spratly Islands in the South China Sea.

Satellite pictures on Google Earth and Google Maps show four large three-pronged structures surrounding a circular building on the western coastline of Itu Aba, or Taiping Island, which is controlled by Taiwan but also claimed by China, Vietnam and the Philippines.

The disclosure of the structures, revealed when images were updated in July, comes amid rising tensions over Beijing’s territorial claims in the South China Sea, and at a sensitive time for the Spratly Islands, a major archipelago located in strategic shipping lanes.

In July, a United Nations court of arbitration upheld a challenge by the Philippines that China could not claim historic rights over the islands. It also characterised Taiping as a rock, meaning it could not be used to assert territorial claims over surrounding waters.

Both China and Taiwan rejected the ruling.

The handling of Taiwan’s control of Taiping over China’s sovereignty claims has become one of the testing grounds of the new administration of President Tsai Ing-wen, who swept to power in January.

On Tuesday, the coast guard director general, Lee Chung-wei, told the national parliament that Google had been notified about concealing the structures.

The defence ministry said the function of the facilities was classified.

“It is inconvenient for us to reveal any military facilities we are installing on Taiping island, and what their purposes are, as they are all considered secret,” the defence minister, Feng Shih-kuan, told reporters.

The purpose of the structures, which are believed to stand three to four storeys high, led to immediate speculation that they resembled anti-aircraft towers. But military experts said they were likely to be for detection and surveillance.

Google confirmed that it had received the request from the Taiwanese government and was reviewing it.