The US vice-president's trip to Japan, Washington's closest regional ally, is meant to "reassure allies" and "deter any escalation".
US Vice President Kamala Harris has condemned "disturbing" actions by China in remarks to American sailors based in Japan, days after the administration pledged US forces will help defend Taiwan if it were attacked.
"China is undermining key elements of the international rules-based order," Harris said in prepared remarks during a visit to the largest overseas US Navy installation in the world at Yokosuka, outside of Tokyo on Wednesday.
"China has flexed its military and economic might to coerce and intimidate its neighbours. And we have witnessed disturbing behaviour in the East China Sea and in the South China Sea, and most recently, provocations across the Taiwan Strait," she added.
The remarks and Harris's trip to Asia come days after US President Joe Biden pledged in an interview aired on September 18 to defend the self-ruled island against an "unprecedented attack."
The US subscribes to a "One China" policy that formally recognises only Beijing, but binds the US government to provide democratically ruled Taiwan with the means to defend itself.
China says Taiwan is one of its provinces. Beijing has long vowed to bring Taiwan under its control and has not ruled out the use of force to do so.
Taiwan's government strongly objects to China's sovereignty claims and says only the island's 23 million people can decide its future.
In the wake of US House Speaker Nancy Pelosi's visit to Taiwan in August, China carried out its largest-ever military exercises around the island.
Harris said US forces would continue to operate in the region "undaunted and unafraid."
"We will continue to oppose any unilateral change to the status quo," she said in the remarks.
"And we will continue to support Taiwan's self-defence, consistent with our long-standing policy. Taiwan is a vibrant democracy that contributes to the global good — from technology to health, and beyond, and the United States will continue to deepen our unofficial ties."
As I told Prime Minister Kishida, the U.S.-Japan Alliance is as strong as it’s ever been. We are committed to strengthening it further, as it is critical to the prosperity and security of the American people. pic.twitter.com/QiBvU1GDVq— Vice President Kamala Harris (@VP) September 26, 2022
Harris's trip to Japan, Washington's closest regional ally, was meant to reassure allies and deter any escalation.
Aides said Harris would work on a unified approach to that challenge within the region, where leaders have warily watched rising tensions between Washington and Beijing.
The base where Harris spoke is home to 24,000 military and civilian workers who could be called on in a regional conflict.
It's also the home of the USS Ronald Reagan, an aircraft carrier now in South Korea to participate in joint drills meant to deter North Korea. Harris will visit the demilitarised zone separating the Koreas on Thursday.
On Tuesday, Harris led Biden's bipartisan US delegation to the state funeral of former Japanese Prime Minister Shinzo Abe, who nudged the country away from the pacifist doctrine it adopted after being defeated in World War Two.
Biden is expected to hold his first face-to-face meeting as president with China's Xi Jinping during November's Group of 20 meeting in Indonesia.