It's a move that's likely to alarm the country's neighbors and further unsettle ties with the United States, where President-elect Donald Trump has shown himself increasingly willing to confront and challenge Beijing.
The images, released by the Asia Maritime Transparency Initiative
, show anti-aircraft guns plus other weapons systems that would guard against cruise missiles sitting in hexagonal structures on the islands.
China has already built military-length airstrips on three of these artificial islands, previous analysis by the AMTI, part of the Washington-based Center for Strategic and International Studies (CSIS), has shown.
China's Defense Ministry said the military installations were proper and lawful, and mainly for self-defense.
"If others are flexing their military muscles at your doorstep, are you not even supposed to have a slingshot in hand just in case?" a statement said.
Since winning the presidency, Trump has seemingly rewritten US foreign policy on the fly.
"All the indications we've seen since the election imply that he will be a more hawkish president vis-a-vis China," said Ashley Townshend, a research fellow at United States Studies Centre at the University of Sydney.
"We don't know yet entirely what his South China Sea policy will be but many of his advisers have been critical of the Obama's administration for being too weak," he added.
The think tank said that the weapon emplacements showed that "Beijing is serious about defense of its artificial islands in case of an armed contingency in the South China Sea."
"Among other things, they would be the last line of defense against cruise missiles launched by the United States or others against these soon-to-be-operational air bases," it added.
Unsinkable aircraft carriers?
Geng Shuang, a spokesman for China's Ministry of Foreign Affairs, said the islands were part of "China's inherent territories" and it was quite normal to deploy "defense facilities" there.
AMTI said the weapons systems observed on the three largest islands -- Fiery Cross, Mischief and Subi reefs -- that China has built in the Spratlys were an evolution of smaller fortifications it had observed on the four other reefs China has reclaimed land at -- Gaven, Hughes, Johnson, and Cuarteron reefs.
"It gives every impression of being a land-based attempt to put close-in weapons systems, that you would normally find on a naval ship, on a concrete superstructure on these features to provide some sort of close-in, last-ditch.... rapid-firing weapons system to try and defeat approaching cruise missiles," retired US Adm. Michael McDevitt of the Center for Naval Analyses told AMTI in a podcast.
Some have called the islands China's "unsinkable aircraft carriers."
China has already deployed surface-to-air missiles on Woody Island
, part of the Paracel island chain -- to the west of the Spratlys in the hotly disputed sea.
US 'ready to confront'
According to the US, China has reclaimed more than 3,000
acres in the Spratly Islands since the beginning of 2014.
Vietnam, Malaysia, the Philippines and Taiwan have also reclaimed land in the South China Sea, but their land grab -- the US says approximately 100 acres over 45 years -- is dwarfed by China's massive, recent buildup.
The Philippines, which lies geographically closest to the Spratlys, has troops stationed in the area, as do other claimants.
The US takes no position on the territorial dispute in the South China Sea but its warships have conducted "freedom of navigation" operations
near the reclaimed islands, eliciting warnings from Beijing.
US Adm. Harry Harris, commander of US Pacific Command, said Wednesday that the US stood ready to confront Beijing over the contested waters.
"I've also been loud and clear that we will not allow the shared domains to be closed down unilaterally - no matter how many bases are built on artificial features in the South China Sea," he said in a speech in Sydney.
"I say this often but it's worth repeating -- we will cooperate where we can and be ready to confront where we must."