The United States warned China on Saturday to halt destabilizing actions in Asia as Washington and its allies sought to boost defense cooperation in the face of what Japan called an "increasingly severe" security environment.
Using unusually strong language, U.S. Defense Secretary Chuck Hagel told an Asia-Pacific security forum that the United States was committed to its geopolitical re-balance to the region and "will not look the other way when fundamental principles of the international order are being challenged".
"In recent months, China has undertaken destabilizing, unilateral actions asserting its claims in the South China Sea," he said in the speech to the Shangri-La Dialogue in Singapore.
Hagel said the United States took no position on the merits of rival territorial claims in the region, but added: "We firmly oppose any nation’s use of intimidation, coercion, or the threat of force to assert these claims."
Japanese Defense Minister Itsunori Onodera said Tokyo perceived an "increasingly severe regional security environment."
"It is unfortunate that there are security concerns in the East and South China Seas," he said. "Japan as well as all concerned parties must uphold the rule of law and never attempt to unilaterally change the status quo by force."
China claims almost the entire oil- and gas-rich South China Seas, and dismisses competing claims from Taiwan, Brunei, Vietnam, the Philippines and Malaysia. Japan has its own territorial row with China over islands in the East China Sea.
Tensions have surged in recent weeks after China placed an oil rig in waters claimed by Vietnam, and the Philippines said Beijing could be building an airstrip on a disputed island.
Japan's defense ministry said Chinese SU-27 fighters came as close as 170 feet to a Japanese OP-3C surveillance plane near disputed islets last week and within 100 feet of a YS-11EB electronic intelligence aircraft.