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."
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.
Kerry hailed Mongolia as an "oasis of democracy" sitting in a tough location between Russia and China as he made a rare visit by a cabinet-level U.S. official on Sunday.
Arriving from Paris, Kerry made the visit to Mongolia en route to Beijing to attend the eighth Strategic and Economic Dialogue, the high-level talks held between the United States and China each year.
The Obama administration has sought to cultivate Mongolia as a partner, given its location between two increasingly assertive U.S. rivals – China and Russia, countries Mongolia relies on heavily for trade and energy imports.
U.S. officials regularly tout Mongolia, a country of three million people, as an inspirational story of democratic transition since winning independence from the Soviet Union a quarter-century ago.
But Mongolia's democratic and human rights records have come under scrutiny.
The 2015 State Department report on human rights in Mongolia noted "vague laws and a lack of transparency in legislative, executive, and judicial processes" that "undermined government efficiency and public confidence and invited corruption."
Parliamentary elections are to be held this month, and democracy advocates have criticized a rule change by parliament last month that is expected to disadvantage smaller parties.
The lack of transparency in business regulations has hampered foreign investment, officials say, badly needed in an economy where growth has all but halted, far from a peak of 17.5 percent in 2011.
Falling prices for its chief exports, coal and copper, and weak demand from China, by far Mongolia's biggest trading partner, are behind the slowdown.
Kerry said it was important the Mongolian government implement a transparency agreement with the United States, which would provide greater business confidence to foreign investors.
The Mongolian and U.S. governments signed the agreement in 2013 after years of negotiations but its final implementation has been mired in red tape.
The agreement would commit Mongolia to publish in English an explanation for proposed regulations, and allow for public comments on those regulations, among other measures.
"The implementation of this agreement is really key, a very important step to be able to attract foreign direct investment," Kerry said.
Kerry will visit China after Mongolia.
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.