U.S. Defense Secretary Chuck Hagel warned China on Saturday that while the U.S. had no position on territorial claims in Asia, it would not "look the other way" if the international order was challenged.
Speaking at an Asia-Pacific security forum in Singapore, Hagel emphasized the strength of the U.S.-Japan alliance and gave a robust defense of U.S. strategic policy in Asia.
His comments come against a backdrop of heightened tension between China, Asia's biggest economy, and its neighbors in the South China Seas.
In a blunt warning to Beijing, Hagel told the gathering of military and government officials that while the U.S. had "no position on territorial claims," it was committed to the freedom of navigation and flights over the South China Sea.
"The United States will not look the other way when fundamental principles of the international order are being challenged," he said.
Calling the South China Sea "the beating heart of the Asia-Pacific and a crossroads for the global economy," Hagel said there was a need to establish a cooperative regional architecture and resolve disputes peacefully.
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He pledged to enhance the defense capabilities of America's regional allies.
China claims almost all of the South China Seas, an area rich in oil and gas, and dismisses rival claims from Taiwan, Brunei, Vietnam, the Philippines and Malaysia.
It is also involved in a territorial spat with Japan over islands in the East China Sea.
Tensions in the past few weeks in particular have soared after China installed an oil rig in waters claimed by Vietnam, sparking anti-China riots in Vietnam's industrial zones.
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The Philippines has said Beijing could be building an airstrip on a disputed island, according to a Reuters report.
Hagel spoke of the long history the U.S. has of engagement with Asia, challenging China to come to the negotiating table, and emphasizing the rising power's relative isolation in comparison to its neighbors.
"Today, America has more peacetime military engagement in the Asia-Pacific than ever," he said. "Our friends and allies can judge us on nearly seven decades of history. As that history makes clear, America keeps its word."
Japan's Prime Minister Shinzo Abe told the Singapore forum on Friday that Tokyo would offer support to countries in Southeast Asian aiming to protect their seas and airspace.
On a trip to Asia last month, U.S. President Barack Obama reassured Asian allies of the U.S. commitment to the region.
In Beijing, President Xi Jinping said China would not initiate aggressive action in the South China Sea but would respond if other countries did, according to a report on Saturday by the official Xinhua news agency.