The United States has warned China to stop using "destabilising" tactics in its bid to claim parts of the South China Sea.
US defence secretary Chuck Hagel made the comments at an Asia-Pacific security forum in Singapore, also attended by Australia's Defence Minister, David Johnston.
China is locked in territorial disputes with Vietnam, the Philippines, Brunei and Malaysia over stretches of the resource rich region.
This month, ships from China and Vietnam clashed near the Paracel Islands and Vietnam said a Chinese ship sunk one of its boats by ramming it.
"In recent months, China has undertaken destabilising, unilateral actions asserting its claims in the South China Sea," Mr Hagel said in the speech to the Shangri-La Dialogue in Singapore.
He said the US had no position on the merits of the rival claims over the region, which is rich in oil and gas.
But he went on to say that aggression would not be ignored.
"We firmly oppose any nation's use of intimidation, coercion, or the threat of force to assert these claims," he said.
Candid comments surprise Chinese officials
The unusually direct comments from the US have surprised Chinese officials in Singapore.
Chuck Hagel later had a bi-lateral meeting with the deputy chief of staff of the Chinese Army, Lieutenant-General Wang Guanzhong.
PHOTO: US defence secretary Chuck Hagel, South Korean defence minister Kim Kwan-jin and Japanese defence minister Itsunori Onodera at the start of the meeting in Singapore. (AFP: Pablo Martinez Monsivais)
"You were very candid this morning, and to be frank, more than our expectations," Wang said at the start of the meeting.
"Although, I do think those criticisms are groundless, I do appreciate your candour," he said.
In Beijing, China's state media is reporting that president Xi Jinping has also responded to the recent criticisms to his nation's action in the South China Sea by saying China would not initiate aggressive action.
"We will never stir up trouble," he is reported to have said.
"But we will react in the necessary way to the provocations of countries involved."
The US is not the only nation at the summit concerned by the recent conflict.
At the forum, Australia also said any use of force or coercion to change the status quo in the South China Sea will not be accepted.
"Australia does not take a position on competing claims in the South China Sea," the Defence Minister, Mr Johnston, said.
"But we have a legitimate interest in the maintenance of peace and stability, respect for international law, unimpeded trade and freedom of navigation.
"The use of force or coercion to unilaterally alter the status quo in the East China Sea and the South China Sea is simply not acceptable."
US warns China that Washington will not remain passive if the international order is threatened - ABC News (Australian Broadcasting Corporation)
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