When President Xi Jinping of China visited President Obama at the White House last September, he startled many with reassuring words about his intentions for the Spratly Islands, a contested area where the Chinese government has been piling dredged sand and concrete atop reefs for the past few years and building housing and runways on them.

“China does not intend to pursue militarization,” Mr. Xi said, referring to the area as the Nansha Islands, a Chinese name for what most of the rest of the world calls the Spratlys in the South China Sea.

The most recent satellite photographs suggest a different plan. The photos, collected and scrutinized by the Center for Strategic and International Studies, a Washington-based research organization, show the construction of what appear to be reinforced aircraft hangars at Fiery Cross, Subi and Mischief Reefs, all part of the disputed territories.

There were no military aircraft seen at the time the photos were taken. But a summary of the center’s analysis suggests that the hangars on all three islets have room for “any fighter-jet in the People’s Liberation Army Air Force.”