A satellite image shows what CSIS Asia Maritime Transparency Initiative says appears to be anti-aircraft guns and what are likely to be close-in weapons systems (CIWS) on the artificial island Hughes Reef in the South China Sea. (Digitalglobe/Reuters)

China is adding new antiaircraft weapons to a string of artificial islands in the middle of the disputed South China Sea, according to a report by U.S. policy-watching groups based on satellite analysis.

The findings, drawn from a study by the Asia Maritime Transparency Initiative and the Center for Strategic and International Studies, appear to challenge vows by China’s leader last year not to “militarize” the area.

In recent years, Beijing has turned scattered coral reefs into small, strategic outposts by piling sand and then building installations such as barracks, lighthouses and military-grade airstrips. China claims sovereignty over the full South China Sea — in direct conflict with international rulings and other countries bordering the sea, including the Philippines and Vietnam.

In a recent television interview, President-elect Donald Trump accused Beijing of building a “massive fortress in the middle of the South China Sea.”

Beijing maintains that these seven artificial islands serve civilian, not military, purposes.

China has laid claim to a number of islands in the South China Sea, building airbases on tiny spits of land while installing powerful radar and missile launchers. Here's why. (Jason Aldag,Julie Vitkovskaya/The Washington Post / Satellite photos courtesy of CSIS)

China’s Defense Ministry said in a statement published Thursday that any “necessary military facilities” were for “defense and self-defense” and were, therefore, “legitimate.”

“If someone shows off his muscles in your doorway,” the statement said, “shouldn’t you at least prepare a slingshot?”

The “muscle,” in this case, is the United States. Though Washington says it has no position on rival sovereignty claims in the South China Sea, the United States has been critical of China’s island-building and conducts patrols — officially called “freedom of navigation operations” — in the area.

China sees the presence of U.S. ships as interference and has repeatedly said that it is the United States, not China, that wants to militarize the South China Sea.

The standoff has been a long-standing source of tension between Washington and Beijing, and nobody seems certain how Trump and his team will handle the dispute.

On one hand, Trump has talked of putting “America first” and focusing more on domestic matters. On the other, his advisers have criticized the Obama administration’s “limited” response to China’s maritime claims.

The Chinese side, for its part, appears to be pushing ahead with its strategy of gradually adding new military features to the artificial islands.

The report by the U.S. groups found that weapons systems spotted on the three largest islands China has built in the Spratlys — Fiery Cross, Mischief and Subi reefs — were an “evolution” of smaller systems previously seen on four other sites in the sea.

The placement of the weapons showed that Beijing “is serious about defense of its artificial islands in case of an armed contingency in the South China Sea,” the report said.

“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.”

There was no immediate U.S. response to the report.

In a speech in Sydney on Wednesday, Adm. Harry Harris, head of the U.S. Pacific Command, suggested that the United States is ready to “confront” China at sea.

“I’ve also been loud and clear that we will not allow the shared domains to be closed down — no matter how many bases are built on artificial features in the South China Sea,” he said.

“I say this often, but it’s worth repeating — we will cooperate where we can and be ready to confront where we must.”

Luna Lin in Beijing contributed to this report.