Stressing US commitments to allies 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.

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 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.

In 2012, the Philippines lost control of rich fishing grounds in Scarborough, about 220 kilometres (135 miles) off its main island, to China after a standoff.

Manila in May publicly accused Beijing of large-scale reclamation activity at Johnson South Reef. Filipino officials fear this could lead to China building its first airstrip in the disputed region.

The Philippines asked a United Nations tribunal in March to declare what Manila said was China's claim to 70 percent of the sea as illegal. Beijing has refused to participate in the tribunal proceedings and repeatedly rejected protests by China and Vietnam.

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".

He pledged support for Japan's plans to play a greater role in maintaining security in Asia.

Japanese Prime Minister Shinzo Abe, opened the forum Friday by saying Tokyo would play a more "proactive" role in Asian security as the leader sets about reshaping the rules for the country's little-used military.

Hagel also pledged support to countries that are moving towards democracy, notably Myanmar, but said Washington would review ties with those curbing democratic freedoms.

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.

Thailand was only represented by a senior foreign ministry official at the Singapore forum.