But analysts note that other countries did not build islands, and that they generally erected their structures before 2002, when China and nine Southeast Asian nations signed the Declaration on the Conduct of Parties in the South China Sea. One clause says the parties must “exercise self-restraint in the conduct of activities” that would escalate tensions and must refrain from inhabiting any currently uninhabited land features.

Although the agreement is nonbinding and does not explicitly ban building on the islands or the creation of new ones, some analysts say those activities are covered.

“It’s changing the status quo,” said Carlyle A. Thayer, an emeritus professor of politics at the University of New South Wales in Australia. “It can only raise tensions.”

Since January, China has been building three or four islands, projected to be 20 to 40 acres each, one Western official said. He added that there appeared to be at least one installation intended for military use, and that the new islands could be used for resupplying ships, including Chinese maritime patrol vessels.

Last month, China set off alarms in the region and in Washington when a state-owned oil company placed an exploratory oil rig farther north in the South China Sea, by the contested Paracel Islands near Vietnam. The rig ignited diplomatic strife and violent anti-China protests in Vietnam.

But the island-building “is bigger than the oil rig,” said the Western official, who spoke on the condition of anonymity to avoid upsetting diplomatic discussions. “These islands are here to stay.”