China's increasing military presence in the disputed sea could effectively lead to a Beijing-controlled air defence zone, they said, ratcheting up tensions with other claimants and with the United States in one of the world's most volatile areas.
Chinese foreign ministry officials confirmed on Saturday that a test flight by a civilian plane landed on an artificial island built in the Spratlys, the first time Beijing has used a runway in the area.
Vietnam launched a formal diplomatic protest while Philippines Foreign Ministry spokesman Charles Jose said Manila was planning to do the same. Both have claims to the area that overlap with China.
"That's the fear, that China will be able take control of the South China Sea and it will affect the freedom of navigation and freedom of overflight," Jose told reporters.
In Washington, State Department spokesman John Kirby said China's landing of the plane "raises tensions and threatens regional stability."
Senator John McCain, the chairman of the influential U.S. Senate Armed Services Committee, criticised the Obama administration for delaying further "freedom of navigation" patrols within 12 nautical miles of the islands built by China.
China has been building runways on the artificial islands for over a year, and the plane's landing was not a surprise.
The runway at the Fiery Cross Reef is 3,000 metres (10,000 feet) long and is one of three China was constructing on artificial islands built up from seven reefs and atolls in the Spratlys archipelago.
The runways would be long enough to handle long-range bombers and transport craft as well as China's best jet fighters, giving them a presence deep into the maritime heart of Southeast Asia that they have lacked until now.
Chinese officials have repeatedly stressed that the new islands would be mostly for civilian use, such as coast guard activity and fishing research.
Foreign ministry spokeswoman Hua Chunying said at the weekend that the test flight was intended to check whether the runway met civilian aviation standards and fell "completely within China's sovereignty".
However, military landings on the islands were now "inevitable", said Leszek Buszynski, a visiting fellow at the Australian National University's Strategic and Defence Studies Centre.
An air defence zone, while unlikely soon, was feasible and possible in future once China's built up its air strength, he said.
"The next step will be, once they've tested it with several flights, they will bring down some of their fighter air power - SU-27s and SU-33's - and they will station them there permanently. That's what they're likely to do."Source: Reuters