Stressing US commitments to allies and friends in Asia, Hagel called for a peaceful resolution of international disputes and issued a blunt message to China, which was represented by a high-level military delegation at the forum.

"In recent months, China has undertaken destabilising, unilateral actions asserting its claims in the South China Sea," Hagel told fellow defence chiefs, military officials, diplomats and security experts attending the annual Shangri-La Dialogue.

He accused China of restricting access by the Philippines to Scarborough Shoal, putting pressure on Manila's long-standing presence in Second Thomas Shoal, beginning land reclamation at various locations and moving an oil rig into disputed waters with Vietnam.

Hagel said that while the United States does not take sides on rival claims, "we firmly oppose any nation's use of intimidation, coercion, or the threat of force to assert these claims".

"The United States will not look the other way when fundamental principles of the international order are being challenged," he said.

Tensions have flared up recently in the South China Sea, claimed almost entirely by China, which has lately taken bold steps to enforce what it says are its historical rights.

Four Southeast Asian states -- Brunei, Malaysia, the Philippines and Vietnam -- claim parts of the sea, with Manila and Hanoi being the most vocal in opposing China's claims. Taiwan is the sixth claimant.