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.
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"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.
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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."
Officials say Johnson South Reef, which China seized in 1988 after killing about 70 Vietnamese soldiers or sailors in a skirmish, is the most developed of the islands so far. "It's Johnson Island now; it's not Johnson Reef anymore," the Western official said. Filipino officials released aerial photographs last month showing structures and a large ship.
Le Hai Binh, a spokesman for the Vietnamese Foreign Ministry, said in an email statement that Vietnam had sovereignty over the entire Spratly archipelago and that "China has been illegally implementing activities of expansion and construction" around Johnson Reef and other sites claimed by Vietnam.
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He said Vietnam demanded that China "immediately stop illegal activities of expansion and construction" on the reef and "withdraw its vessels and facilities from the area."
The Spratlys comprise hundreds of reefs, rocks, sandbars and tiny atolls spread over 160,000 square miles. Six governments have overlapping claims in the area. China and Vietnam also have competing claims for the Paracel Islands, in the area where the Chinese oil rig still sits. Both areas have abundant fish and some oil and gas reserves.
Jin Canrong, a professor of international studies at Renmin University of China, said he believed that the construction on Johnson South Reef was "a technical test, to see if such things can be done." Should China want to try island-building on a larger scale, he said, a logical choice would be Fiery Cross Reef, about 90 miles west of Johnson South.
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Last month, digital sketches of structures intended for the Spratlys circulated on Chinese news websites, including that of Global Times, a newspaper owned by People's Daily, the Communist Party mouthpiece. The sketches, labeled a research study, showed a new island with shipping docks, parking lots and an airfield with a runway, airplanes and hangars. Reports said the images were from the China Shipbuilding NDRI Engineering Company, in Shanghai. When asked about the sketches over the phone, a woman at the company said they were "too sensitive" and had been taken off the firm's website. She declined to comment further.
Wu Shicun, president of the National Institute for South China Sea Studies, a government-linked research group on Hainan Island, said Chinese construction was intended mainly to augment the country's fisheries administration and humanitarian relief capabilities, not for military purposes.
"Our facilities are worse than those of both the Philippines and Vietnam," he said. "You see that Vietnam even has a soccer field."
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Vietnamese and Filipino naval personnel played soccer during a June 8 conclave on Southwest Cay Island, which is controlled by Vietnam. "Clearly this was meant to enrage the Chinese people," Mr. Wu said. The island has been occupied by the Vietnamese military since the 1970s but is also claimed by China and the Philippines.
Christopher K. Johnson, the chief China analyst at the Center for Strategic and International Studies, in Washington, said China's recent moves were partly to make up for the fact that the Chinese military focused mainly on Taiwan for more than a decade while Vietnam and the Philippines developed facilities on shoals and reefs they controlled.
He said Chinese military officials were probably keeping in mind future long-range naval power projections. "There's no doubt that they would love to have some kind of a naval facility on one of these things," he said.
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Chinese military leaders have talked in recent years of building up a navy that can operate beyond what is commonly called the "first usland chain" — islands closer to mainland Asia that include the Spratlys and Paracels — to penetrate the "second island chain," which includes Guam and other territories farther east.
Mr. Thayer, the Australian analyst, said he had seen no signs yet that China was building large military facilities or a runway on the new islands. But he said there was a clear conclusion to be drawn from China's actions in both the South China Sea and the East China Sea, where China contends with Japan over islands.
"None of this is an isolated incident," he said. "It seems to be a new plan to assert Chinese sovereignty. This isn't something that will go away. This is a constant thing that will raise tensions, and at the same time no one has a good response to it."