The US defence secretary, Chuck Hagel, has said China actions in the South China Sea are "destabilising" and "unilateral", rubbishing Beijing's description of the waters as a "sea of peace, friendship and cooperation".
Hagel made the remarks on Saturday at an Asian security summit in Singapore, during a time of increasing maritime tension between China and its neighbours including Japan, the Phillipines and Vietnam.
He said: "China has called the South China a 'sea of peace, friendship and cooperation'. And that's what it should be.
"But in recent months, China has undertaken destabilising, unilateral actions asserting its claim in the South China Sea.
"It has restricted access to Scarborough Reef, put pressure on the long-standing Phillipine presence at the Second Thomas Shoal, begun land reclamation at multiple locations, and moved an oil rig into disputed waters near the Paracel Islands."
The statement came a day after the US delivered the first of a fleet of Global Hawk drones to Japan.
While the US took no position on competing territorial claims, he told the audience: "We oppose any nation's use of intimidation, coercion or the threat of force to assert those claims".
"The United States will not look the other way when fundamental principles of the international order are being challenged."
China was quick to react to Hagel's speech. The deputy chief of the general staff of the People's Liberation Army called the remarks baseless, the AFP news agency reported.
"This speech is full of hegemony, full of incitement, threats, intimidation," said Wang Guanzhong, who is due to make his own speech at the summit on Sunday.
"Moreover [it] is public, several times criticising China by name, and these kinds of accusations are completely without basis, without reason."
Wang's tone was markedly different from that of China's president Xi Jinping who, on Friday, promised not to "stir up trouble" in the South China Sea and would only "react as necessary" to the provocations of other countries involved.
Beijing's decision to deploy an oil platform in waters claimed by Vietnam provoked anti-Chinese riots, with thousands of Chinese citizens being evacuated from Vietnam as a result.
At the same Singapore-based security summit Japan's prime minister, Shinzo Abe, said on Friday that his country wanted to play a greater role in promoting peace and prosperity in the Asia-Pacific region.
Abe said efforts "to consolidate changes to the status quo by aggregating one fait accompli after another can only be strongly condemned".
He did not identify China by name, but praised the Philippines and Vietnam for their efforts to resolve the disputes through dialogue.