American Navy has now conducted four so-called freedom-of-navigation operations in the past year in the disputed waters.
China appears to have installed weapons – including anti-aircraft and anti-missile systems – on all seven of the artificial islands it has built in the South China Sea.
Citing new satellite imagery, the Asia Maritime Transparency Initiative (AMTI), a US think-tank, said on Wednesday that its findings come despite statements by the Chinese leadership that Beijing has no intention of militarising the islands in the strategic trade route, where territory is claimed by several countries.
AMTI said it had been tracking construction of hexagonal structures on Fiery Cross, Mischief, and Subi reefs in the Spratly Islands since June and July. China has already built military-length airstrips on these islands.
Satellite images 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, it said.
“These gun and probable CIWS emplacements show that Beijing is serious about defence of its artificial islands in case of an armed contingency in the South China Sea,” AMTI said.
“Among other things, they would be the last line of defence against cruise missiles launched by the United States or others against these soon-to-be-operational air bases.”
AMTI director Greg Poling said his group spent months trying to figure out what the purpose of the structures was.
“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 news agency.
“This is militarisation. 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.
“They keep saying they are not militarising, but they could deploy fighter jets and surface-to-air missiles tomorrow if they wanted to,” he said. “Now they have all the infrastructure in place for these interlocking rings of defence and power projection.”
Al Jazeera’s Adrian Brown, reporting from Beijing, said the development was significant because China repeatedly insisted that its activities in the South China Sea was not for military purposes.
“China insists these islands are there to assist fishermen in distress and also for scientists to monitor the maritime and ecological life in the region,” he said.
“These islands lie on the most important trading routes in the world, making them very strategic.”
The US has 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.
US President-elect Donald Trump, who takes office on January 20, has also criticised Chinese behaviour in the South China Sea, while signalling that he may adopt a tougher approach in the region than President Barack Obama.