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

A satellite image released by the Asian Maritime Transparency Initiative at Washington's Center for Strategic and International Studies shows construction of possible radar tower facilities in the Spratly Islands. Photo taken February 2016. CSIS Asia Maritime Transparency Initiative/DigitalGlobe

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

The United States has previously criticized what it called China’s militarization 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.

China’s Defence Ministry said in a statement on its website on Thursday that the construction it had carried out on islands and reefs in the disputed Spratlys chain was “mainly for civilian use”.

“As for necessary military installations, they are mainly for defence and self-protection and are legitimate and lawful,” it said. “If someone makes a show of force at your front door, would you not ready your slingshot?”

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.

“PREPPING FOR CONFLICT”

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 (CIWS) to protect against cruise missile strikes.

Other images showed towers that 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.

AMTI director Greg Poling said the think-tank had spent months trying to figure out the purposes of the structures shown in the images.

“This is the first time that we’re confident in saying they are anti-aircraft and CIWS emplacements. We did not know that they had systems this big and this advanced there,” he told Reuters.

“This is militarization. The Chinese can argue that it’s only for defensive purposes, but if you are building giant anti-aircraft gun and CIWS emplacements, it means that you are prepping for a future conflict.”

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

“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 militarization, then what is the sailing of fleets into the South China Sea?”

REGIONAL CONCERN

The Philippines, one of several countries with competing territorial claims in the South China Sea, said it was still verifying the report.

“But if true it is a big concern for us and the international community who use the South China Sea lanes for trade,” said Defence Minister Delfin Lorenzana. “It would mean that the Chinese are militarising the area which is not good.”

Lorenzana’s comments were made during a visit to Singapore with President Rodrigo Duterte, where he also said the United States had agreed to sell the Philippine Navy two advanced radar systems to boost its surveillance capability in the South China Sea.

Australia too voiced concerns about China’s actions in the disputed waterway.

“The building of artificial islands and the possible militarization is creating an environment of tension and mistrust between claimants and other regional states,” said Foreign Minister Julie Bishop in a statement.

“We urge claimants to refrain from coercive behavior and unilateral actions designed to change the status quo in disputed areas.”

U.S. President-elect Donald Trump, who takes office on Jan. 20, has criticized Chinese behavior in the South China Sea, while signaling he may adopt a tougher approach to China’s assertive behavior in the region than President Barack Obama.

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 militarization of disputed features.”