Satellite images released by the Center for Strategic and International Studies in Washington this week showed “large antiaircraft guns and probable close-in weapons systems” on its outposts in the Spratlys.

“As for necessary military facilities, they are primarily for defense and self-protection, and this is proper and legitimate,” the Chinese Defense Ministry said on its website in response to the report, which was made by the group’s Asia Maritime Transparency Initiative. “For instance, if someone was at the door of your home, cocky and swaggering, how could it be that you wouldn’t prepare a slingshot?”

The comments left little doubt that such installations were part of China’s plan to deepen its territorial claim over the islands, which has raised tensions with its neighbors and Washington over freedom of navigation in the South China Sea, one of the world’s busiest commercial waterways.

They were also very likely to further complicate China’s already testy relations with the incoming administration of President-elect Donald J. Trump. China’s rapid creation of artificial islands in the South China Sea, expanding former reefs and outcrops into guarded permanent outposts, has already become a major source of tension with Washington.

Mr. Trump recently angered Chinese officials by holding a phone conversation with Tsai Ing-wen, the president of Taiwan, an island that Beijing deems an illegitimate breakaway from mainland China. Mr. Trump’s predecessors, as president-elect or in office, did not have such direct contact with Taiwanese leaders for nearly four decades.

In an interview broadcast on Sunday, Mr. Trump also criticized China over its trade imbalance with the United States, its military activities in the South China Sea and its ties to North Korea. China was “building a massive fortress in the middle of the South China Sea, which they shouldn’t be doing,” he said in the interview on Fox News.

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

“Among other things, they would be the last line of defense against cruise missiles launched by the United States or others against” air bases that may soon go into operation on the islands, it said. The images showed that the facilities were in place before Mr. Trump’s comments.

The latest images also increased the uncertainty over the intent of comments made by China’s president, Xi Jinping, after meeting President Obama in the White House in September last year. The Obama administration has said it does not take a position on who has sovereignty over the disputed islands across the South China Sea, but it wants to protect freedom of navigation there and defuse conflict.

With Mr. Obama at his side, Mr. Xi told reporters that “China does not intend to pursue militarization” of the islands, which China calls the Nansha Islands. Previous satellite pictures of the islands, released in August, already indicated that China was building military facilities there. Those images appeared to show reinforced aircraft hangars at the Fiery Cross, Subi and Mischief Reefs, all part of the disputed parts of the archipelago.

But on Thursday, a spokesman for China’s Ministry of Foreign Affairs defended his government’s right to take such steps and said they did not count as “militarization.”

The spokesman, Geng Shuang, said he could not confirm the findings of the Center for Strategic and International Studies’ report. But he added, “China deploying necessary territorial defensive facilities on its own territory has nothing to do with so-called ‘militarization.’”