US defense secretary Chuck Hagel today accused China of "destabilising" the South China Sea, saying its action threatened the region's long-term progress.
Hagel said the US would "not look the other way" when nations ignored international rules, while speaking at the three-day Shangri-La Dialogue in Singapore that involves the US and South-East Asian countries.
He also urged Thailand's coup leaders to restore democratic rule ''soon''.
The forum comes amid growing tensions between China, Vietnam and the Philippines, with Japan-China ties also strained over disputed islands in the East China Sea.
The summit gives senior delegates from the region a chance to meet face-to-face to try to resolve tensions.
"In recent months, China has undertaken destabilising, unilateral actions asserting its claims in the South China Sea," Hagel said in his address.
Chinese and Vietnamese vessels have confronted each other in disputed waters in the South China Sea.
"We firmly oppose any nation's use of intimidation, coercion, or the threat of force to assert these claims," he added, referring to the way China has claimed territorial rights over areas of the South China Sea close to Japan, the Philippines and Vietnam.
"All nations of the region, including China, have a choice: to unite, and recommit to a stable regional order, or to walk away from that commitment and risk the peace and security that has benefited millions of people."
He said he supported Japan's offer to play a greater and "more proactive" role in regional security, as promised by Japanese Prime Minister Shinzo Abe in his keynote speech on Friday.