US Defense Secretary Chuck Hagel has accused China of "destabilising actions" in the South China Sea.
He warned that Washington will not remain passive if the international order is threatened.
Stressing US commitments to allies and friends in Asia, Secretary Hagel called for a peaceful resolution of international disputes.
He 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," Mr Hagel told fellow defense chiefs, military officials, diplomats and security experts attending the annual Shangri-La Dialogue in Singapore.
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.
Mr 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.
A Chinese military official has blasted the United States for making "threats".
The Chinese army's deputy chief of staff, Wang Guanzhong, described Secretary Hagel's comments at the Shangri-La Dialogue as baseless.
"Secretary Hagel's speech is full of threats and intimidating language.
"Secretary Hagel's speech is full of encouragement, incitement for the Asia region's instability giving rise to a disturbance," state broadcaster China Central Television quoted Mr Wang as telling reporters.
"Secretary Hagel, in this kind of public space with many people, openly criticised China without reason. This accusation is completely without basis," Mr Wang added.