There was no immediate comment from Mr. Trump or his transition team.

Mr. Trump recently angered Chinese officials by holding a phone conversation with Tsai Ing-wen, the president of Taiwan, an island that Beijing deems a breakaway province of China. It had been nearly four decades since a United States president or president-elect had such direct contact with a Taiwanese leader.

In an interview broadcast on Sunday, Mr. Trump also criticized China over its trade imbalance with America, 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.

During his campaign, Mr. Trump dwelled on accusations that China had systematically sapped American industrial might, and he has indicated that trade issues will be a priority in dealings with Beijing. But the latest disclosures suggest how seemingly remote islands in the South China Sea could become a source of serious tensions, even military strife.

The Spratlys, which China calls the Nansha Islands, are the subject of an especially volatile mix of competing claims. Parts of the archipelago are also claimed by the Philippines, Malaysia, Vietnam, Brunei and Taiwan. And the possibility of undersea oil and gas deposits has exacerbated the rivalries.

President Rodrigo Duterte of the Philippines has moved in recent months to ease tensions with China, and to distance his country from Washington. Even so, the Philippines keeps defense treaties with the United States.

But China, with the world’s second-biggest economy and a swelling military budget, has established an intimidating dominance across much of the South China Sea. And the latest satellite images appeared to confirm its deepening military grip on the Spratlys.