US Defense Secretary Chuck Hagel warned Beijing Saturday against "destabilising actions" in the South China Sea, and backed its regional rival Japan's plans to take on a more muscular security role in Asia.

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 Singapore.

"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 the Philippines' access 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.

China reacted angrily to Hagel's comments, with Deputy Chief of the General Staff of the People's Liberation Army (PLA) Wang Guanzhong describing them as baseless, and further condemning him for making them in public.

"This speech is full of hegemony, full of incitement, threats, intimidation," Wang was quoted as saying by a reporter from state broadcaster China Central Television.

"Moreover (it) is public, several times criticising China by name, and these kinds of accusations are completely without basis, without reason," Wang said. The military official is due to make his own speech on Sunday.

Tensions have recently flared up 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.

In the latest outbreak of tensions, Vietnam accused Chinese warships Thursday of pointing weapons at their vessels during an escalating standoff near an oil rig in contested waters. There have also been ramming incidents involving boats from both sides lately.

The Philippines and China are locked in a bitter dispute over the control of islets and reefs in the sea, which straddles vital shipping lanes and is believed to sit atop vast gas deposits.

Manila publicly accused Beijing in May of large-scale reclamation activity at Johnson South Reef which it fears could be an airstrip development.

China is also in dispute with Japan over islands in the East Sea, which Tokyo calls Senkaku and Beijing refers to as Diaoyu.

Last year, China declared an air defence identification zone in the East Sea, including over the outcrops, which are under Japan's administration.

In his speech, Hagel reiterated that the United States opposes "any effort by any nation to restrict overflight or freedom of navigation, whether from military or civilian vessels, from countries big or small".

Restating a US declaration, he said "the Senkaku Islands fall under our mutual defence treaty with Japan" and backed Tokyo's plans to play a greater role in maintaining security in Asia.

Japanese Prime Minister Shinzo Abe opened the Singapore forum Friday by saying his country would play a more "proactive" role in Asian security, including providing coast guard vessels to the Philippines and potentially Vietnam.

Hagel also pledged support to countries that are moving towards democracy, notably Myanmar.

He urged the Thai military junta that took power in a coup on May 22 to release people they have detained, end restrictions on freedom of expression, and "move immediately" to hold elections.

Hagel said that until that happens, the Pentagon will continue to suspend and reconsider US military assistance and engagements with Thailand.

Despite tough words for China's behaviour in disputed Asian waters, Hagel also cited efforts to forge a "new model of relations" between Beijing and Washington, including military cooperation.

"The United States is reaching out to China -- because we seek to expand prosperity and security for all nations of the region," he said.

He noted that Chinese ships will next month join for the first time the annual US-led Rim of the Pacific maritime exercise -- the world's largest -- with 23 nations, 49 surface vessels, six submarines, more than 200 aircraft and 25,000 personnel taking part.