China has landed a plane for the first time on one of its new island runways in the disputed part of South China Sea.
Vietnam said the plane landed on Jan. 2 and 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.
China has confirmed 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.
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 Senate Armed Services Committee, criticized the Obama administration for delaying further "freedom of navigation" patrols within 12 nautical miles of the islands built by China.
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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 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".
Asked about McCain's remarks on Tuesday, she said: "We hope the U.S. can take an objective and fair attitude, and not make statements that confuse the situation and are harmful to regional peace and stability."
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China claims most of the South China Sea, through which more than $5 trillion of world trade ships every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.
The United States has no claim in the South China Sea, but has been highly critical of China's assertiveness and says it will protect freedom of navigation.