Pentagon chief Lloyd Austin said Tuesday that Beijing's expansive claims in the South China Sea have "no basis in international law", taking aim at China's growing assertiveness in the hotly contested waters.

"That assertion treads on the sovereignty of the states in the region," he said at the start of a trip to Southeast Asia, where several countries have competing claims with China in the sea.

"We continue to support the region's coastal states in upholding their rights under international law."

Speaking in Singapore, he said the US "will not flinch when our interests are threatened, yet we do 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."

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