US Vice President Kamala Harris has hit out at China during a speech made in Singapore on the first leg of a South East Asian tour, reports BBC.
Ms Harris accused Beijing of coercion and intimidation in the South China Sea, which has been a regional flashpoint for years.
She said the US would "stand with our allies in the face of threats".
Ms Harris' trip is seen as an attempt to reaffirm US commitment to the region.
Her speech which touched on a number of other issues, also mentioned the US pull-out from Afghanistan, where she insisted the the decision was "courageous and right".
On China, Ms Harris criticised its claims "to the vast majority of the South China Sea," which she said were based on intimidation and coercion.
"These unlawful claims have been rejected by the 2016 arbitral tribunal decision and Beijing's actions continue to undermine the rules-based order and threaten the sovereignty of nations," she said.
Ms Harris was referring to a landmark legal victory the Philippines won over China, concerning territorial incursions in the South China Sea. Since 2012, and despite the tribunal ruling, there has been a constant Chinese Coastguard presence there - with Filipino fishermen reporting harassment by the authorities.
In recent years, China has been increasingly assertive over what it says are centuries-old rights to the contested region, and has been rapidly building up its military presence to back up those claims.
Several other countries including Japan claim ownership of various small islands and reefs that dot the sea and with it, access to resources.
Although the South China Sea is a massive regional issue, Ms Harris' trip has been largely overshadowed by the country's rushed exit from Afghanistan, which is due to happen by 31 August.
She did not go into detail on the issue which has been criticised by the global community, only saying the US had "achieved what we had gone there to do", and adding that they were now "laser focused" on evacuating Afghans who had worked with US troops.
The Taliban's swift takeover of Afghanistan in just 10 days has shocked the world.
Comparisons have been made with the so-called fall of Saigon in Vietnam 1975.
Ms Harris is due to visit Vietnam later on Tuesday.