Beijing regards the islands as its sovereign territory and says it is entitled to necessary defensive installations.

China defended its right on Thursday to put “necessary military installations” on artificial islands in the South China Sea, after a US think-tank said Beijing appeared to have deployed weapons such as anti-aircraft and anti-missile systems.

The Asia Maritime Transparency Initiative (AMTI) at the Center for Strategic and International Studies said its findings, made available first to Reuters news agency on Wednesday, were based on an analysis of satellite images of islands in the strategic trade route, where territory is claimed by several countries.

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The United States has conducted four freedom of navigation patrols – seen as a challenge to China’s extensive territorial claims in the South China Sea – in the past year or so, most recently in October.

“As for necessary military installations, they are mainly for defence and self-protection and are legitimate and lawful,” China’s Defence Ministry said in a statement on its website.

“If someone makes a show of force at your front door, would you not ready your slingshot?”

The United States has previously criticised what it called China’s militarisation of its maritime outposts, and stressed the need for freedom of navigation by conducting periodic air and naval patrols near them that have angered Beijing.

The statement said the construction it had carried out on islands and reefs in the disputed Spratlys chain was “mainly for civilian use”.

AMTI said satellite images of islands China has built in the Spratlys showed what appeared to be anti-aircraft guns and what were likely to be close-in weapons systems to protect against cruise missile strikes.

Other images showed towers that most likely contained targeting radar, it said.

Beijing regards the islands as its sovereign territory, and has often said it is entitled to limited and necessary defensive installations.

Chinese Foreign Ministry spokesman Geng Shuang told a regular news briefing in Beijing he “did not understand” the situation referred to in the AMTI report.

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“The Nansha islands are China’s inherent territory. China’s building of facilities and necessary territorial defensive facilities on its own territory is completely normal,” he said, using China’s name for the Spratlys.

“If China’s building of normal facilities and deploying necessary territorial defensive facilities on its own islands is considered militarisation, then what is the sailing of fleets into the South China Sea?”

US president-elect Donald Trump, who takes office on January 20, has criticised Chinese behaviour in the South China Sea, while signalling he may adopt a tougher approach to China’s assertive behaviour.

The State Department said it would not comment on intelligence matters, but spokesman John Kirby added: “We consistently call on China – as well as other claimants – to commit to peacefully managing and resolving disputes, to refrain from further land reclamation and construction of new facilities, and the militarisation of disputed features.”