US: China's sea claims have 'no basis' in international law

Austin's broadside came at the start of his first trip to Southeast Asia as US defence secretary, as he seeks to rally allies in the region as a bulwark to China.

President Joe Biden's administration wants to reset relations with Asian countries and build alliances to face Beijing, after the turbulence and unpredictability of the Donald Trump era.

Speaking in Singapore, Austin criticised China's actions in the disputed sea, where Beijing has overlapping territorial claims with several Southeast Asian states. 

"Beijing's claim to the vast majority of the South China Sea has no basis in international law," he said in a speech hosted by the International Institute for Strategic Studies think tank.

"That assertion treads on the sovereignty of the states in the region," he said, adding that the US would support countries in defending their rights.

China claims almost all of the resource-rich sea, through which trillions of dollars in shipping trade passes annually, with competing claims from Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

Beijing has been accused of deploying a range of military hardware including anti-ship missiles and surface-to-air missiles there, and ignored a 2016 international tribunal decision that declared its historical claim over most of the waters to be without basis.

Tensions have escalated in recent months between Beijing and rival claimants.

Manila was angered after hundreds of Chinese boats were spotted inside the Philippines' Exclusive Economic Zone, while Malaysia scrambled fighter jets to intercept Chinese military aircraft that appeared off its coast.

Austin said Tuesday that the US "will not flinch when our interests are threatened" -- but he insisted Washington does "not seek confrontation" with China.

"I am committed to pursuing a constructive, stable relationship with China, including stronger crisis communications with the People's Liberation Army."