Her comments in Singapore came as Washington seeks to reset relations in Asia after the turbulent Donald Trump era and build a bulwark against the rising might of Beijing.

But her trip to the region, which also includes a stop in Vietnam, comes as Washington faces fresh questions over its dependability amid the US pullout from Afghanistan and Taliban takeover.

In a speech laying out her administration's foreign policy goals, Harris reiterated that Washington had "enduring commitments" in Asia -- and took aim at China.

"Beijing continues to coerce, to intimidate and to make claims to the vast majority of the South China Sea," she said.

"Beijing's actions continue to undermine the rules-based order and threaten the sovereignty of nations."

Read also: US, China trade barbs at UN over South China Sea

But Beijing hit back, holding up the Afghan debacle as an example of the United States' "selfish" foreign policy, and accusing Washington of "bullying, hegemonic behaviour".

"The current events in Afghanistan clearly tell us what the rules and order the US speaks of are," China's foreign ministry spokesman Wang Wenbin said.

China claims almost all of the resource-rich South China Sea, through which trillions of dollars in shipping trade passes annually, with competing claims from four Southeast Asian states as well as Taiwan.

Beijing has been accused of deploying a range of military hardware including anti-ship 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 recently between Beijing and rival claimants, with Manila angered after hundreds of Chinese boats were spotted at a contested reef while Malaysia scrambled jets to intercept Chinese military aircraft.

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