Vice President Kamala Harris accused China on Tuesday of intimidation in disputed Asian waters, seeking to rally regional allies as US superpower status takes a hit over Afghanistan.
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, Ms Harris said 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.”
But Beijing hit back, holding up the Afghan debacle as an example of “selfish” US 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 South-East Asian states as well as Taiwan.
Beijing has been accused of sending a range of military hardware including anti-ship and surface-to-air missiles there, and ignoring a 2016 international tribunal decision that declared its historical claim over most of the waters to be without basis.
Tension has 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.
Ms Harris also sought to allay fears that growing US-China tension could force countries that have strong ties with both of the world's top economies to choose sides.
“Our engagement in South-East Asia and the Indo-Pacific is not against any one country, nor is it designed to make anyone choose between countries,” she said.
The US vice president is the latest top official from President Joe Biden's administration to visit the region. On a trip to Singapore last month, Pentagon chief Lloyd Austin also strongly criticised China's maritime claims.
But the Afghan crisis and doubts about US credibility are casting a long shadow over Ms Harris's visit.
In Tuesday's speech, she defended Mr Biden's decision to push ahead with the US pullout from Afghanistan as “courageous and right” and said US officials were “laser-focused” on the chaotic evacuation of Kabul airport.
“The United States has been focused on safely evacuating American citizens, international partners, Afghans who worked side by side with us and other Afghans at risk,” she said.
Ms Harris also announced that Washington was offering to host the annual meetings of the Asia-Pacific Economic Co-operation (Apec) forum in 2023, a group that includes Beijing and the US.
The US-China relationship has deteriorated due to a range of issues, including cyber security, tech supremacy and human rights in Hong Kong and Xinjiang.
Mr Biden has largely continued Mr Trump's hawkish stance on China, describing the Asian power as the pre-eminent challenge to the US, but has lowered the temperature.
The vice president also called for greater international co-operation to strengthen supply chains during talks with Singapore officials and business executives, as the US seeks to tackle a global microchip shortage.
She arrived in Hanoi late on Tuesday but has faced criticism for going ahead with the Vietnam leg of the visit amid the Afghanistan evacuation.
That crisis has prompted comparisons with Saigon in 1975, when US helicopters ferried final evacuees from the embassy roof.
Ms Harris's trip from Singapore was delayed due to an "anomalous health incident" in Hanoi, the US embassy said, an apparent reference to the so-called "Havana syndrome" that has sickened diplomats in several countries.
"The vice president's travelling delegation was delayed from departing Singapore because the vice president's office was made aware of a report of a recent possible anomalous health incident in Hanoi," the US embassy statement said.