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.