U.S. Secretary of State John Kerry said on Sunday the United States would consider any Chinese establishment of an air defense zone over the South China Sea to be a "provocative and destabilizing act".
U.S. officials have expressed concern that an international court ruling expected in coming weeks on a case brought by the Philippines against China over its South China Sea claims could prompt Beijing to declare an air defense identification zone, or ADIZ, as it did over the East China Sea in 2013.
"We would consider an ADIZ...over portions of the South China Sea as a provocative and destabilizing act which would automatically raise tensions and call into serious question China's commitment to diplomatically manage the territorial disputes of the South China Sea," Kerry said during a visit to Mongolia.
"So we urge China not to move unilaterally in ways that are provocative."
Kerry will visit China after Mongolia.
China drew condemnation from Japan and the United States when it imposed its ADIZ, in which aircraft are supposed to identify themselves to Chinese authorities, above the East China Sea.
China has neither confirmed nor denied it plans such a zone for the South China Sea, saying that such a decision would be based on the threat level and that it had every right to set one up.
China claims most of the South China Sea through which trillions of dollars in ship-borne trade passes every year and has been undertaking extensive reclamation and construction activities on islands and reefs it occupies.
Neighbors Brunei, Malaysia, the Philippines, Taiwan and Vietnam also have claims.
Chinese dredging vessels are purportedly seen in the waters around Fiery Cross Reef in the disputed Spratly Islands in the South China Sea in this still image from video taken by a P-8A Poseidon surveillance aircraft provided by the United States Navy May 21, 2015. U.S. Navy/Handout via Reuters/File Photo