The Asia Maritime Transparency Initiative (AMTI) said its findings come despite Beijing stating it has no intention to militarise the Spratly Islands.
AMTI said it had been tracking construction of hexagonal structures on Fiery Cross, Mischief and Subi reefs since June and July.
China has already built military length airstrips on the islands, which lie in a strategic trade route where territory is claimed by several countries.
“It now seems that these structures are an evolution of point-defence fortifications already constructed at China’s smaller facilities on Gaven, Hughes, Johnson, and Cuarteron reefs,” it said, referring to newly released images taken in November.
“This model has gone through another evolution at (the) much-larger bases on Fiery Cross, Subi and Mischief reefs.”
Satellite images of Hughes and Gaven reefs 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, AMTI said.
Images from Fiery Cross Reef showed towers that probably contained targeting radar.
AMTI said covers had been installed on the towers at Fiery Cross, but the size of platforms on these and the covers suggested they concealed defence systems similar to those at the smaller reefs.
“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,” it 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.”
Chinese foreign ministry spokesman Geng Shuang told a news conference in Beijing that he “did not understand” the situation referred to in the report.
“The Nansha (Spratly) 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.
China’s defence ministry said it was “legitimate and lawful” for it to place defensive military installations on islands where it had “indisputable sovereignty”.
“If someone makes a show of force at your front door, would you not ready your slingshot?” it said.
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 Lorenza.
“It would mean that the Chinese are militarising the area, which is not good.”