The comments came after images released by the US-based Asia Maritime Transparency Initiative (AMTI) showed a series of hexagonal structures on each of seven islets China constructed on uninhabitable rocks and reefs.
The structures appear to be large anti-aircraft guns and close-in weapons systems (CIWS) designed to take out incoming missiles and enemy aircraft, the AMTI said.
Speaking at a regular press briefing, foreign ministry spokesman Geng Shuang told journalists that he was not aware of the report, but said that islands in the South China Sea were the country's "inherent territory".
China's defence ministry said on a verified social media account that the construction was mostly for civilian use, and that necessary military installations were for self-defence.
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"If someone were flexing his muscles outside your door, wouldn't you get a slingshot ready?", it wrote.
Tensions in the strategic area have been brewing for years, with China, the Philippines, Brunei, Malaysia and Vietnam making competing claims in waters with vital global shipping routes and what is believed to be significant oil and natural gas deposits.
If true, "then it is a cause for serious concern because it tends to raise tension and undermine peace and stability in the region," spokesman Charles Jose told AFP.
In a statement sent to AFP, Australia's foreign minister Julie Bishop said that the building of artificial islands and possible militarisation was "creating an environment of tension and mistrust" in the region.