US vice president Kamala Harris has delivered a sharp rebuke to China for its incursions in the South China Sea, warning its actions there amount to “coercion” and “intimidation” and affirming that the US will support its allies in the region against Beijing’s advances.

“We know that Beijing continues to coerce, to intimidate and to make claims to the vast majority of the South China Sea,” she said in a major foreign policy speech Tuesday in Singapore in which she laid out the Biden administration’s vision for the Indo-Pacific. “Beijing’s actions continue to undermine the rules-based order and threaten the sovereignty of nations.”

China claims almost all of the resource-rich 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 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 in recent months between Beijing and rival claimants.

Harris’s speech sought to cement the US commitment to supporting its allies in an area of growing importance to the Biden administration, which has made countering China’s influence globally a centrepiece of its foreign policy.

Harris, who is on a week-long tour of south-east Asia, declared that the US “stands with our allies and our partners” in the face of threats from China.

Harris’s remarks come during a critical moment for the United States as the Biden administration seeks to further solidify its pivot towards Asia while America’s decades-long focus on the Middle East comes to a messy end with the chaotic withdrawal from Afghanistan.

The speech echoed and expanded upon remarks she delivered at the US Naval Academy graduation in June, where she described a world that is “interconnected”, “interdependent” and “fragile”.

On Monday, Harris told sailors aboard a US combat ship at the Changi naval base in Singapore that “a big part of the history of the 21st century will be written about this very region” and that their work defending the region was pivotal.

“It is in our vital interest to stand united with our allies and our partners in south-east Asia in defence of a free and open Indo-Pacific,” she said.

On Monday, Harris met Singapore president Halimah Yacob and prime minister Lee Hsien Loong. The vice president’s office announced a number of agreements out of that meeting aimed at combating cyberthreats, tackling climate change, addressing the Covid-19 pandemic and alleviating supply chain issues.

Next, Harris is to hold a roundtable discussion with business leaders on supply chain issues, and then travel to Vietnam, where she’ll meet senior officials on Wednesday.

President Joe Biden himself has repeatedly emphasised his focus on China as one of America’s main adversaries, pledging in a February speech at the state department to “confront China’s economic abuses; counter its aggressive, coercive action; to push back on China’s attack on human rights, intellectual property, and global governance” – a message he echoed again later that month to European allies at the virtual G7 summit and Munich security conference.

And in recent months, his administration has ramped up outreach to the Indo-Pacific region, with defence secretary Lloyd Austin and deputy secretary of state Wendy Austin both visiting the area. Secretary of state Antony Blinken also held a number of virtual meetings with south-east Asian officials this month.

The chaotic US withdrawal from Afghanistan, however, complicated that message of support to the region, raising questions about the US commitment to its allies. While Biden said last week that an indefinite engagement would have benefited “true strategic competitors” China and Russia, China has seized on the images of violence from the evacuation to criticise the US for its engagement there.

Chinese foreign ministry spokesperson Wang Wenbin said on Monday that the US had done “unscrupulous and dishonest things” in Afghanistan and called on it to help rebuilding efforts. “The United States is the root cause and the biggest external factor in the Afghan issue,” Wenbin said. “It cannot just run away like this.”

But Harris, during a joint news conference with Lee on Monday, said her presence in the country, combined with the agreements around greater cooperation that the Biden administration has pursued with Indo-Pacific countries, spoke “volumes in terms of the integrity of the relationships that the United States has around the world on many issues”.

With Associated Press and Agence France-Presse